The degradation of 4-hydroxyazobenzene and Solvent Red 1 on various oxide particles (SiO2, Al2O3 and TiO2) was monitored using diffuse reflectance spectroscopy. Adsorption of the dye onto the support material can facilitate photolytic degradation by affecting its bonding. Analysis of the absorption spectra throughout the degradation process indicates that the C-N bond in these azo dyes is the site for oxidative attack. The dependence of degradation rate on surface coverage indicates that direct interaction between the dye molecule and the TiO2 particles is necessary for sensitized photocatalysis to be an effective method of degradation.