Highly selective photodecarboxylation could be achieved by utilizing a series of two-component molecular crystals of aralkyl carboxylic acids such as 3-indolepropionic acid (a) and 1-naphthylacetic acid (b) combined with acridine (1) or phenanthridine (2) as an electron acceptor. The 1:1 hydrogen bonded crystals were prepared by recrystallization from the solutions. Irradiation of the crystals at -70 degrees C caused highly selective decarboxylation to give corresponding decarboxylated compounds alone in nearly quantitative yields, due to a smaller thermal motion of the radical species in the crystal lattice. Upon irradiating a crystal, a carboxylate radical and hydroacridine or hydrophenanthridine radical are produced via electron transfer from the acid to 1 or 2 and subsequent proton transfer followed by decarboxylation. Next hydrogen abstraction by an active aralkyl radical occurs in highest priority over the shortest distance of 3.2-3.5 Angstrom resulting in the formation of a corresponding decarboxylated product and the regeneration of 1 or 2. Occurrence of radical coupling is low due to the longer coupling distance of 4.5-6.5 Angstrom estimated from the crystallographic data of the starting two-component molecular crystals. The hydrogen abstraction path in the crystal lattice could be confirmed by the reaction of a deuterated crystal 1.bD in which the CO2H group was replaced by CO2D, giving a deuterated l-methyl(CH2D)naphthalene as a product. Regeneration of 1 or 2 means that the acceptor plays the role of a stoichiometrical sensitizer, which can act in only one cycle, retaining the initial crystal structure. Such a stoichiometrical sensitization is a novel photochemical process, which occurs specifically in the solid state.