The branching ratio of the excited-state population at the conical intersection between the S-1 and S-0 energy surfaces (Phi(CI)) of a protonated Schiff base of all-trans retinal in protic and aprotic solvents was studied by multipulse ultrafast transient absorption spectroscopy. In particular, pump-dump-probe experiments allowed to isolate the S-1 reactive state and to measure the photoisomerization time constant with unprecedented precision. Starting from these results, we demonstrate that the polarity of the solvent is the key factor influencing the Phi(CI), and the photoisomerization yield.