Poor aqueous solubilities of drug candidates limit their bioavailability. A number of delivery systems are in use to enhance the bioavailability of the drugs with poor solubility in water. The self-assemblies of hydrotropes, surfactants and oil/water micro emulsions may provide a means of enhancing solubility and enhance bioavailability of drugs. Although these drugs delivery systems are in use but the mechanism through which these delivery systems solubilize the drugs needs detail investigations. The objective of the current dissertation was to provide the understanding of the mechanism through which simple aggregates of hydrotropes, micelle of surfactants and oil in water microemulsions solubilize the drugs. For the purpose, apparent solubility of drugs namely, Meloxicam, Celecoxib, Ibuprofen and Lidocaine was determined in aqueous solution of hydrotropes, surfactants, surfactant/hydrotrope, surfactants/butanol mixtures and in oil/ water microemulsions. These mediums were tested for their ability to enhance the aqueous solubility of these water insoluble drugs. The results obtained for molar solubilization ratio (MSR), partition co-efficient (K M ) of the investigated drugs concluded that these were lower in hydrotropes as compared to the one obtained in other stated systems. Among the hydrotropes, sodium benzoate showed highest (0.006- 0.0107), whereas sodium p-toluene sulfonate (0.0014- 0.0052) the lowest MSR values. The negative values obtained for ∆G o illustrated the spontaneous mixing of these drugs in all the investigated systems. The CMC, HLB, oxyethylene units and aggregation number of surfactants along with molecular mass of the drug, polarity of the drug and the group attached to them showed a great impact over the solubility of two model drugs, Meloxicam and Celecoxib in nonionic surfactants including Tween 20, Tween 80, Brij 30, Brij 35, Triton X-100, xiv and Triton X-114. It was noted that the surfactants with high aggregation number solubilized higher amount of drugs and had higher value of MSR than others. The solubility was enhanced with the increase in number of oxyethylene units in a surfactant. The solubility was also increased with the increase in number of carbon atoms in alkyl chain of surfactants used. Similar results were observed when Lidocaine was solubilized in ionic, nonionic and zwitterionic surfactants. Among the nonionic, N,N, Dimethyledodecyle amine-N-Oxide (DDAO) whereas among ionic and zwitterionic surfactants, N,N, Dimethyldodecyle- amonio propane-sulfonate (DDAPS) surfactants showed higher ability to solubilize the model drug, Lidocaine. The addition of hydrotropes and/or butanol to aqueous solution of DDAPS showed a noticeable increase in solubility of all the investigated drugs. In case of oil/ water microemulsion, the increase in molecular mass of oil in a homologous series increased the solubility of drugs. It was also noticed that microemulsions had highest ability to solubilize the drugs among all the investigated systems. The results obtained by light scattering revealed that the addition of drugs does not increase the aggregation number and hydrodynamic radius of the surfactants micelles. However, both the aggregation number and size was increased by the addition of butanol and hydrotropes. The addition of hydrocarbon to the DDAPS/butanol mixture resulted a decrease in micellar size as well as the aggregation number. Similar observations were also made for aggregation number and hydrodynamic radius in case of Menthol or Eutectic mixtures of Lidocaine/Menthol used as an oil phase. All these observations concluded that the drugs are solubilized in inner core of micelles/aggregates of the surfactants/hydrotropes. However in case of oil/water microemulsions these were solubilized only in oil phase of microemulsions.