The factors influencing the encapsulation of azathioprine (AZA) into liposomes were investigated to find out the conditions for its optimal entrapment. Similar studies for comparison were also carried out on 6-mercaptopurine (6-MP), of which AZA is a prodrug. AZA and also 6-MP show higher encapsulation efficiencies in MLVs as compared to LUVs. Variation in phospholipid composition does not seem to affect the loading capacity of either of the two drugs. The encapsulation efficiency of both the drugs improves upon addition of cholesterol in the bilayer, but the effect is seen only up to 30% cholesterol. Thereafter the effect becomes constant. AZA shows better incorporation in the positively charged liposomes as compared to those with neutral or negative charge. The entrapment of 6-MP is, however, found to be independent of the charge on the liposomes. Entrapment efficiency for both the drugs markedly depends on the pH of the hydration medium, yielding better entrapment efficiencies at high pH values. The rise in solute concentration initially causes increase in the entrapment of the two drugs which is followed by a decreasing phase.