Syntheses based on physical methods, such as pressure and light, are extremely attractive to prepare novel materials from pure molecular systems in condensed phases. The structural and electronic modifications induced by selective optical excitation can trigger unexpected chemical reactions by exploiting the high density conditions realized at high pressure. The identification of the microscopic mechanisms regulating this reactivity, mandatory to design synthetic environments appealing for practical applications, requires a careful characterization of both structural and electronic properties as a function of pressure. Here, we report a spectroscopic study, by FTIR and Raman techniques, of the ambient temperature photoinduced reactivity of liquid C(2)H(5)OD up to 1 GPa. The results have been interpreted by comparison with those relative to the fully hydrogenated isotopomer. The dissociation along the O-H (D) coordinate is the primary reactive channel, but the different reactivity of the two isotopomers with rising pressure highlights a dramatic pressure effect on the energy surface of the first electronic excited state. Dissociation along the O-H (D) coordinate becomes the reaction rate-limiting step due to an increase with pressure of the binding character along this coordinate.