In this work, to understand how an amorphous surface influence the dynamics of surface photoinduced reactions, pump-probe spectroscopy in conjunction with mass spectrometry is employed to track the ultrafast evolution of intermediates and final products with time, mass, and energy resolution. As a model system, the photoinduced reaction of CD3I adsorbed on amorphous cerium oxide films is investigated. A fraction of the first intermediates produced on a freshly prepared surface are trapped to passivate the surface. After the A-band excitation, the minimum dissociation time of CD3I, indicate that CD3I adsorption geometries with either CD3 or I facing the gas-phase exist, however the transient data suggest that most molecules are adsorbed with the I atom facing the surface. CD3 and I are consumed to form I2 and reform CD3I, which are produced with a steady rate only after the intermediates are losing the excess translational energy released from photodissociation.