Emission from cis-1-(2-anthryl)-2-phenylethene, c-APE*, in toluene is resolved into (1)t-APE(B)* and (1)c-APE* components at temperatures ranging between 4.3 and 59.3 degrees C. Decomposition of effective fluorescence quantum yields, <(phi)over bar>(fc), into pure component fluorescence quantum yields, phi(ft-B) and phi(fc) shows that phi(ft-B) increases 24% with increasing temperature while phi(fc) decreases more than 3-fold over this temperature range. On the basis of the fraction of molecules that escape the (1)c-APE* potential energy minimum, 1 - phi(fc), the efficiency of adiabatic formation of (1)t-APE(B)* remains remarkably temperature independent at 50.5 +/- 0.7%. These results, together with photoisomerization quantum yields as a function of [c-APE] in degassed and air-saturated toluene, reveal a detailed photoisomerization mechanism. At infinite dilution and in the absence of molecular oxygen, photoisomerization of c-APE occurs predominantly via the adiabatic, conformer-specific (1)c-APE(B)* --> (1)t-APE(B)* pathway. This torsional motion experiences a 4.4(4) +/- 0.1(4) kcal/mol barrier probably located at the perpendicular, (3)p*, geometry. Since 12% of (1)t-APE(B)* intersystem cross to (3)t-APE(B)*, the known triplet state quantum chain process enhances photoisomerization quantum yields at higher [c-APE]. Triplets formed directly from (1)c-APE* also contribute to this pathway. In air-saturated solutions, oxygen eliminates the quantum chain process by reducing the lifetime of (3)t-APE*. However, the quenching of (1)c-APE* by O-2 gives (3)c-APE*, thus enhancing photoisomerization quantum yields via rapid (3)c-APE* --> (3)t-APE* adiabatic torsional displacement. No photoisomerization of (1)c-APE(A)* need be postulated to account for our observations. The enthalpy difference between ground state conformers, Delta H-AB, favors C-APE(B) by 0.92 +/- 0.02 kcal/mol.