The photochemistry of diethyl E,E-2,3-(3,4,5-trimethoxybenzylidene)succinate (8) is solvent dependent. In both protic and aprotic solvents, there is a photoequilibrium established between 8 and its E,Z-isomer (9). In chloroform at high light intensity, very little 9 is formed and the main product is 1,4-dihydronaphthalene (10), formed via photoinduced intramolecular [1,3]-sigmatropic hydrogen shift within an intermediate 1,8a-dihydronaphthalene (11). In protic solvents, irradiation of either 8 or 9 ultimately gives primarily the cis-1,2-dihydronaphthalene product (13), along with smaller amounts of the trans isomer (14). By using deuterated solvents, it was shown that 13 and 14 are formed by solvent protonation (or deuteration) of the 1,8a-dihydronaphthalene intermediate (11 or 12).