AbstractPhotochromic measurements are reported on the decay of the coloured form of 1′,3′,3′‐trimethyl‐6‐nitro[2H‐(1)‐benzospiropyran‐2,2′‐indoline] dispersed in poly(methyl methacrylate) and poly(ethyl methacrylate) matrices. Polarized light was used to photoselect molecules in specific orientations, and the data are analysed in terms of a free volume distribution controlling the kinetic matrix effect. A time correlation function is obtained which characterizes both the rotation and the isomeriz