Earlier herein, we irradiated hexane solutions of isoprene (15 wt%) to enable a comparison with the corresponding results for 1,3-butadiene. We now tested different concentrations of isoprene solutions (5 -60 wt.% in hexane) keeping a constant photosensitizer concentration (0.1 mol%) to explore how dilution affects the dimerization yield. Low isoprene concentrations are interesting as they resemble the dilute solutions that result when photobiologically produced isoprene is trapped. As seen in Fig. , the yield of dimers decreases linearly with decreased isoprene concentrations. isoprene in hexane as a function of 1,1-dinaphthylmethanone concentration (0.1 -0.5 mol%).