The photosensitized dimerization of 1,3cyclohexadiene in solution has been shown to be viscosity dependent, with an increase in the yield of dimers as the pressure, and consequently, the viscosity of the solvent, increases. The decay from the T1 state also has a dependence on the solvent viscosity at low viscosity ranges, behaving as a diffusion-limited process. For this reason, we tested solvents with different viscosities (hexane (0.3 cP), cyclohexane (0.1 cP), tetradecane (2.3 cP), and hexadecane (3.4 cP)), and carried out irradiation experiments (24 h) on solutions of isoprene in these solvents (15 wt%) and with 0.1 mol% of 1,1-dinaphthylmethanone. The relative yields of dimers were assessed by 1 H NMR spectroscopy (Table , Fig. , ESI), based on the ratio of the integration of protons from the starting material isoprene and the protons of the dimers formed. In contrast to earlier observations by Liu et al., our results show that the yield of isoprene dimers is indeed affected by a change in solvent viscosity, however, there is not a linear dependence on the viscosity over the range we chose. In fact, the yield decreases when going from hexane (0.3 cP) to cyclohexane (1.0 cP) and tetradecane (2.3 cP), but slightly increases again when going to hexadecane (3.4 cP). Just like the T1 decay dependence upon solvent viscosity, the yield of isoprene dimers is viscosity-dependent only for a low range of viscosity values. Among the ones studied herein, hexane is clearly the optimal solvent with a somewhat higher dimerization efficiency than the other alkane solvents.