Experiments involving picosecond stimulated-emission spectroscopy have monitored the resonance emission following vibronic excitation of the 1:1 complexes of perylene with n-hexane, n-octane and the three isomers each with 1-chlorobutane and 1-chloropentane. The experiments confirm that photoisomerization is occurring in the alkyl halide complexes, by labelling the transient resonant emission. The rates of vibrational relaxation (IVR) at 355 cm - 1 excess energy differ by up to a factor of three for different isomers, and are independent of isomeric stability.