Fluorescence decay curves for 5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine tetraanion (TPPS4-) have been measured in the absence and presence of the methylviologen dication (MV2+) with various ionic strengths in methanol. In the presence of MV2+ the fluorescence decays can be expressed by a double exponential function, I(t) = I(1)exp(-t/tau (1)) + I(2)exp(-t/tau (2)). The contribution by the faster decay component to the total fluorescence signal increases with increasing MV2+ concentration. The faster decay process is attributed to fluorescence from the excited state of a solvent-separated ion pair (SSIP) formed between TPPS4- and MV2+, and the slower process is attributed to fluorescence from free TPPS4- ions in the solution. Rate constants for the quenching of fluorescence from free TPPS4- by MV2+ (k(q)) and formation constants for the SSIP (K-SSIP) were calculated and both are found to decrease with increasing ionic strength. The decrease in k(q) and K-SSIP values can be interpreted in terms of the shielding of electrostatic attraction between the ions. (C) 2001 Elsevier Science B.V. All rights reserved.