Studies of the degradation of olanzapine bulk drug under hydrolytic (acidic and alkaline), oxidising, and photolytic conditions are reported in this paper. Olanzapine is almost completely stable under photolytic and hydrolytic (acidic and alkaline) conditions. The major degradation products formed when olanzapine bulk drug was exposed to oxidative stress conditions were isolated by preparative reversed-phase highperformance liquid chromatography and characterized by FTIR, LC-MS-MS, and 1H, 13C, and DEPT NMR. On the basis of results from spectroscopy the degradation products were characterized as 10-hydroxy-2-methyl-5,10-dihydro-4H-benzo[b]thieno [2,3- e][1,4]diazepin-4-one, 2-methyl-5,10-dihydro-4H-benzo[b]thieno[2,3-e][1,4]diazepin-4- one, and 2-methyl-10-(2-methyl-10H-benzo[b]thieno[2,3-e][1,4] diazepin-4-yloxy)-5,10- dihydro-4H-benzo[b]thieno[2,3-e]diazepin-4-one. Identification, isolation, and characterization of these oxidative degradation products are discussed in detail.