This study investigated TiO2 photocatalytic degradation of caffeine, a stimulating drug, in aqueous suspensions of titanium dioxide under a variety of conditions. Pure TiO2 powders were prepared using a modified homogeneous-precipitation process at low temperature (HPPLT) method. The degradation was studied by monitoring the intermediates employing high performance liquid chromatography (HPLC) separation coupled to an atmospheric pressure ionization mass spectrometry (API-MS) system operated under selected ion monitoring (SIM). These results indicate the original concentration of caffeine almost completely degraded within 360 min, and a degradation ratio of more than 50 % appearing within 120 min of irradiation in a pH range of 3—10. The reaction rates for the decomposition of caffeine in water are higher for runs at pH 3 (k = 0.013 min-1) and pH 11 (k = 0.012 min-1). The proposed conversion mechanism of caffeine was used. The caffeine was first oxidized to become dimethyl parabanic acid and further degraded to di(hydroxymethyl) parabanic acid. This study also detected the product from N-demthylation, theophyline.