A theoretical model is developed in this work to simulate a solar still connected to an external solar collector and incorporating Sodium Thiosulfate Pentahydrate as phase change material (PCM). Excellent agreement is obtained between the theoretical and experimental results for the unit productivity of fresh water and the basin water temperature profile. Sodium thiosulfate pentahydrate worked well as PCM to supply energy during night time for continuous water production and has improved the overall productivity. Incorporating large PCM mass in the system reduced the productivity; increasing the ratio of PCM mass to water mass from 10 to 100% reduced the productivity by up to 30%. While, incorporating large PCM mass in the system has kept the basin water temperature higher for longer time. This is useful if the unit is to be used for supplying potable water and hot water mainly after sunset and for heating purposes especially in cold places. Reducing the overall heat transfer coefficient from 10.4 to 2.6 W/m(2).K can increase the productivity by > 100%. The cooling water through the glass cover can have significant effect (37% increase) on the unit productivity of fresh water if its flow rate is increased from 0.01 to 0.1 kg/s.