X-ray and γ-ray irradiation of benzene solutions of cis- or trans-stilbene causes interconversion towards a stationary cis→trans mixture. The yield of interconversion is smaller for cis→trans than for trans→cis. Both yields increase with stilbene concentration up to a limiting G(trans→cis) of about 2.5 at 0.1M, which corresponds to G(excited stilbene) of about 5. At low stilbene concentrations the yield is lowered by oxygen and by anthracene and enhanced by naphthalene, triphenylene, phenanthrene, and diphenyl. Diacetyl and p-terphenyl preferentially enhance the trans→cis conversion. The results are explained on the basis of primary excitation of solvent benzene which forms excited stilbene by energy transfer, as in photosensitized isomerization. Direct excitation and possibly charge recombination may account for the primary solvent excitation. Enhancement by additives is due to energy transfer from excited benzene to form eventually triplet additive, which, owing to its much longer lifetime, has a greatly improved chance to transfer its energy to stilbene, at lower concentrations of the latter. A detailed study was made of the change of G(isomerization) with the concentration of stilbene (5×10−5 to 10−1M) and of naphthalene (0–7.8M). For 0.1M trans-stilbene, G(formation of excited stilbene) is about 5 in benzene and about 6 in molten naphthalene. Cyclohexene, used as a specific scavenger of triplet benzene, retards the isomerization by about one-third, while nitrous oxide, used as an electron scavenger, has no effect. Isomerization yields in toluene are higher than in benzene, while those in methylcyclohexane are much lower. Addition of naphthalene strongly increases the isomerization yields in both solvents, just as in benzene. No significant temperature dependence was found. X rays at 200 kV have only about 60% of the efficiency of 60Co gamma rays.