Pesticides contamination is a wide spread source of environmental pollution and a matter of concern all over the world. In recent decades, the use of pesticides for agricultural purposes has steadily increased. Surface and ground water contamination due to extensive use of pesticides is a serious threat to the environment and human health. Pesticides are very toxic compounds causing various diseases. Therefore, in this research work, efforts have been made to determine various chlorinated pesticides contamination in water samples of selected areas of Khyber Pukhtoon Khwa, Pakistan. The determination of 20 chlorinated pesticides have been performed in surface and ground water samples collected from different districts (Peshawar, Mardan, Sawabi and Charsadda) of Khyber Pukhtoon Khawa, Pakistan. A total of 239 water samples (157 surface and 82 ground water samples) were collected from various locations of selected areas. The number of surface water samples, collected from Peshawar, Mardan, Charsadda and Sawabi districts were 59, 42, 30 and 26 samples, respectively. Similarly, 29, 24 and 29 ground water samples were collected from Peshawar, Charsadda and Mardan districts, respectively. Solid phase microextraction (SPME) technique was used for various pesticides extraction from water samples. Chlorinated pesticides were determined in water samples using SPME coupled with gas chromatography and electron captured detector (SPME-GC-ECD) method. Using this method, the plots of chlorinated pesticides concentrations and peak areas showed good linear correlation with R2 values in the range of 0.9887 to 0.9999. The percent recovery found at 1 μg L-1 and at 2 μg L- 1 concentration of pesticides ranged from 89.9 ± 3.9 % to 106.0 ± 2.3 and 88.5 ± 2.6 to 109.2 ± 2.9 %, respectively. The lower limits of detection for all 20 organochlorine pesticides were found significantly less than their maximum contamination levels. Among the various pesticides analyzed, β-BHC, γ-BHC, heptachlor, aldrin, α- chlordane, endosulfan I, 4,4''-DDE and dieldrin were detected in most of the water samples. However, other pesticides, like heptachlor epoxide, γ-chlordane, endrin, 4,4''-DDD, endrin ketone, 4,4''-DDT, methoxychlor, α-BHC, δ-BHC, endosulfan I, endosulfan II, endrin aldehyde and endosulfan sulfate were mostly present in only few water samples in the present investigation. Chlorpyrifos and monocrotophos are very toxic organophosphorous pesticides and have more importance due to their extensive use, large distribution and persistence in the environment. They are very dangerous by all routs of absorbtion, such as inhalation, ingestion and skin contact and producing various diseases. In the present study, the irradiation of chlorpyrifos and monocrotophos in aqueous solution by gamma-rays were investigated on a laboratory scale and their degradation efficiency was studied. The GC-ECD and HPLC (High performance liquid chromatography) were used for analysis of chlorpyrifos and monocrotophos, respectively. Aqueous solutions of different concentrations of chlorpyrifos compound (200–1000 μg L-1) were irradiated to different doses (30–575 Gy). Gamma irradiation showed 100 % degradation efficiency (η) for a 500 μg L-1 solution at an absorbed dose of 575 Gy. The dose rate of source was 300 Gy h -1. The radiolysis kinetics for decay of chlorpyrifos was pseudo-first order. The dose constants determined in this study ranged from 8.2 ´ 10-3 to 2.6 ´ 10-2 Gy−1, and decreased with an increase in the initial concentration of chlorpyrifos. The radiation chemical yield (G-values) for the loss of chlorpyrifos was found to decrease with increasing absorbed dose. The gamma radiation induced decomposition of monocrotophos aqueous solution at various concentrations (60-150 mg L-1) was carried out and its removal efficiency (η) was investigated. At 1200 Gy absorbed dose of gamma irradiation, monocrotophos solution (60 mg L-1) showed 100% degradation. The dose constants investigated in this study ranged from 1.4 ´ 10-3 to 3.0 ´ 10-3 Gy-1. The monocrotophos solution prepared in tap water showed lower degradation than that solution prepared in Milli Q water. The effect of radical scavengers, such as tert-butanol, iso-propanol, H2O2, CO3-2, HCO3-, humic acid, NO3- and NO2- as well as saturation of solutions by N2, N2O or air on the degradation of chlorpyrifos and monocrotophos were also studied. The results showed that the oxidative hydroxyl radical (•OH) was the most important in the degradation of chlorpyrifos and monocrotophos, while the reductive radicals, aqueous electron (e-aq) and hydrogen radical (H•) were of less relative importance for the degradation of these two pesticides. The inorganic by-products of chlorpyrifos (Cl -, SO 4-2 and PO4 -3) and monocrotophos (NO3-, NH4+ and PO4-3) were quantitatively determined by ion chromatography (IC). The organic intermediates of monocrotophos degradation were determined by HPLC/MS and UPLC/MS/MS. A detail mechanism pathway for degradation of monocrotophos by gamma irradiation has been proposed. Keywords: Organochlorine pesticides; Chlorpyrifos; Monocrotophos; Gamma irradiation; SPME-GC-ECD; Advanced oxidation technologies; Ion chromatography; Water decontamination; UPLC/MS/MS; Degradation mechanism.