Introduction: Exclusive breastfeeding during the first six months of life is the ideal form of infant feeding. Health benefits of exclusive breastfeeding to infant and mother are well documented in literature. Despite this, the rate of exclusive breastfeeding in the first six months of infants‘ life in Kenya remains low. National nutrition strategy for maternal, infant and young child has set its 2017 target for exclusive breastfeeding at 80% from 32%. To achieve this target, new strategies are needed to complement current exclusive breastfeeding initiatives. Application of theory of planned behaviour model was used to determine the psychosocial factors associated with exclusive breastfeeding and also to identify modifiable and inexpensive interventions to promote exclusive breastfeeding. Objective: The primary objective of this study was to determine the modifiable psychosocial factors associated with exclusive breastfeeding. The secondary objective was to determine the effect of maternal demographic characteristics on exclusive breastfeeding attitude, subjective norm and perceived behaviour control scores. Study design: This was a prospective cohort study to determine psychosocial factors associated with exclusive breastfeeding using theory of planned behaviour. The study was conducted from October 2013 to March 2014. Methods: The study was undertaken at the antenatal clinic in Pumwani Maternity Hospital. It included all pregnant women who were ≥18years, at ≥ 36 weeks gestation and attending Pumwani Maternity Hospital antenatal clinic. A quantitative theory of planned behaviour based questionnaire was used to collect data from the women who fit the inclusion criteria at baseline after they had signed a written consent. Aggregate scores for each of the exposure categories were compiled to determine whether the women had positive or negative attitude, subjective norm and perceived behavioural control. Those with score above zero were considered to have positive psychosocial scores and those with scores below zero were considered to have negative psychosocial scores. Study women were followed up by a telephone interview at two and four months to determine whether they were exclusively breastfeeding or using mixed feeds for their newborns. A second household member was contacted to verify the mothers report. Results: A total of 200 women were recruited to the study at baseline. Mothers included in the final analysis were 180 while 20 were lost to follow up. Proportion of mothers exclusively breastfeeding at four months was 45.6% (95%CI 38.4-52.9). Psychosocial factors associated with exclusive breastfeeding were maternal attitude and perceived behavioural control over exclusive breastfeeding. Mothers who had positive