The degradation of four pharmaceuticals, carbamazepine (CBZ), diclofenac (DCF), sulfamethoxazole (SMX) and trimethoprim (TMP) by ozonation was studied under a range of experimental conditions, including ozone dosage and concentration of target compounds. The concentration profile of the pharmaceuticals and detection of any by-products formed was carried out using liquid chromatography mass spectrometry. CBZ, DCF, TMP and SMX at initial concentration of 5 mg/L each were degraded to below the method detection limit (1 μg/L) when they reacted with 1.6, 2.3, 2.8 and 4.5 mg/L of ozone, respectively. For each parent compound several by-products were detected after the ozone treatment. A number of these by-products have not been previously reported in the literature. Some of these by-products were founds to be quite resistant to ozone up to applied ozone dosages of 15 mg/L.