The photophysical properties of a ring B benzoporphyrin derivative 1,3-diene dimethylester were investigated in solution (water-DMSO mixtures). At low proportions of water in DMSO, the diester obeys Beer's law at all pH values and ionic strengths investigated. For solvent systems containing more than 30% water, the extent of deviation from Beer's law as a function of porphyrin concentration is attributed to formation of porphyrin dimers. Dimerization equilibrium constants (Kd) have been determined in different percentage of solvent mixtures, using absorption and fluorescence spectroscopies, and were found to exhibit a linear correlation between log (Kd) and the inverse of the dielectric constant (log (Kd) = 24.6 - 1060.4 x 1/epsilon (mix)). This equation permits the calculation of Kd in mixtures of water-DMSO.