An experimental approach, employing cholesteric liquid crystalline solvents, has allowed the preferred orientations of collisions leading to photodimerization of acenaphthylene to be determined. The distribution of the syn and anti dimers of acenaphthylene and dimerization quantum yields have been determined in toluene, n-butyl stearate, and a 1/1 mixture of 5α cholestan-3βyl acetate and 5αcholestan-3βyl nonanoate as a function of temperature, solvent phase, and acenaphthylene concentration. Quantum yields have been measured in compensated nematic and cholesteric mixtures of cholesteryl chloride and cholesteryl nonanoate, and in the cholestanyl ester mixture in the presence of tetralin. Large enhancements of the photodimerization rate were found only for irradiations conducted in cholesteric phases. For instance, the quantum yield for photodimerization of 0.02 M acenaphthylene at 35 °C is 0.76 in a 1/1 mixture of 5αcholestan-3βyl acetate-5αcholestan-3βyl nonanoate and is 0.012 in toluene. An explanation of the results which includes the effects of solvent order on the orientations of solute-solute collisions is advanced. The isomeric distribution of photodimers is much less dependent on solvent order than on acenaphthylene concentration and solvent viscosity. Factors which perturb the solvent order and, therefore, alter the reaction efficiency are discussed.