We systematically examined the mechanism of the solvent polarity dependence of the fluorescence ON/OFF threshold of the BODIPY (boron dipyrromethene) fluorophore and the role of photoinduced electron transfer (PeT). In a series of BODIPY derivatives with variously substituted benzene moieties at the 8-position, the oxidation potential of the benzene moiety became more positive and the reduction potential of the BODIPY fluorophore became more negative as the solvent polarity was decreased; consequently, the free energy change of PeT from the benzene moiety becomes larger in a more nonpolar environment. Utilizing this finding, we designed and synthesized a library of probes in which the threshold of fluorescence ON/OFF switching corresponds to different levels of solvent polarity. These environment-sensitive probes were used to examine bovine serum albumin (BSA) and living cells. The polarity at the surface of albumin was concluded to be similar to that of acetone, while the polarity of the internal membranes of HeLa cells was similar to that of dichloromethane.