The decay processes involved in the photochemical double bond isomerization of styrene are documented by means of MC-SCF computations. Possible intersystem crossing (ISC) and internal conversion (IC) pathways have been studied by geometry optimization of the lowest points on the potential energy surface crossings and computation of the spin-orbit coupling constants. The isomerization of beta-methylstyrene (1-phenylpropene) proceeds (Lewis, F. D.; Bassani, D. M. J. Am. Chem. Sec. 1993, 115, 7523-7524) via temperature-independent and temperature dependent pathways in solution. The temperature-independent isomerization process is consistent with a reaction path that begins with ISC at an S-1/T-2 crossing which occurs at the planar S-1 minimum. The lowest-energy S-1/S-0 crossing minimum (conical intersection) is benzene-like, and will not lead to isomerization. Rather, the second temperature-dependent isomerization mechanism also begins with ISC either at the twisted S-1 minimum (also an S-1/T-2 crossing) or at the planar S-1 minimum after adiabatic cis-trans isomerization on S-1 has occurred. Decay from T-2 to S-0 takes place via a T-2/T-1 conical intersection, followed by one of two different T-1/S-) crossing points : the expected twisted T-1 minimum, or a higher-energy benzene-like structure. Because of the large energy gap, S-1--> S-0 IC at the twisted S-1 minimum is unlikely to take place as previously suggested.