The temperature dependence of the singlet state lifetime and photoisomerization and fluorescence quantum yields for trans- and cis-1-phenylpropene have been determined in hexane solution. Calculated barriers for twisting about the double bond on the singlet potential energy surface are 8.8 and 4.6 kcal/mol for the trans and cis isomer, respectively. The barrier for the trans isomer is sufficiently high to prevent isomerization on the singlet state surface at or below room temperature. However, isomerization occurs at low temperatures as a consequence of intersystem crossing to the triplet state, which undergoes barrierless isomerization. The quantum yield for intersystem crossing, as determined by time-resolved photoacoustic calorimetry, is 0.60 +/- 0.03 and the rate constant for intersystem crossing is 4.7 x 10(7) s(-1). While internal conversion is not significant at or below room temperature, thermally activated internal conversion competes with singlet isomerization at high temperatures. The cis isomer undergoes isomerization predominantly via the singlet state at room temperature. Both electron-donating (p-methoxy) and electron-withdrawing (m- and p-cyano, p-carbomethoxy, and p-trifluormethyl) aromatic substituents are found to lower the barrier for singlet state isomerization. Increased solvent polarity (acetonitrile vs hexane) results in variable decreases in the barrier for singlet state isomerization. Photoisomerization of the p-cyano derivative at room temperature occurs predominantly via the triplet state in hexane solution and via the singlet state in acetonitrile solution. The effects of substituents and solvent are better correlated with the magnitude of the S-2-S-1 energy gap than the stability of either zwitterionic or biradical intermediates. Rate constants for intersystem crossing are, in most cases, not highly dependent upon aromatic substitution or solvent polarity.