Hydrocarbons formation following UV photo-induced dissociation of CD3Cl trapped and caged inside thin amorphous solid water (ASW) layers on Ru(001) has been measured for the first time under well-defined UHV conditions. Stable products such as C2D6, CHD3, CD3CD2Cl, CD3OH were detected via post-irradiation temperature programmed desorption. Specific reactivity pathways for the various photo-products were identified based on excitation wavelengths, ASW layer thickness, and parent molecules initial coverage dependence. Cross sections of (1-6) x 10(-19) cm(2) and (1-3) x 10(-20) cm(2) at 193 nm and 248, respectively, were measured. These photo-induced phenomena of caged molecules are discussed as a possible mechanism for the formation of hydrocarbons in interstellar space. (C) 2003 American Institute of Physics.