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The paper focuses on two aspects of photochemical processes in condensed phase: (i) the mechanism of the reaction; (ii) the role of electron-phonon coupling in determining the photoreactivity in condensed phase. Phonon spectroscopy can conveniently be used to investigate both these aspects. In this paper results will be presented from the study of photodimerization of p-formylcinnamic acid (p-FCA) and dimethylated benzoquionones. The system p-FCA is a protosystem, especially in view of the fact that the presence of water influences the course of photochemical reaction. Laser Raman phonon spectroscopy is used to determine if the reaction proceeds by a homogeneous or a heterogeneous mechanism. The role of strong electron-phonon coupling in localizing the excitation and, thus, assisting the reactivity is investigated by low temperature electronic spectroscopy.