The formation of the complex between photogenerated magnesium octaethylporphyrin cation and hydroxide has been studied in mixed alcohol/water solvents. This complex has been postulated as a participant in the transport of hydroxide and/or hydrogen ion across bilayer lipid membranes. The absorption spectrum of the complex (lambda/sub max/=665nm) is red shifted from that of the cation (lambda/sub max/=680nm) in both methanol/water (M/W) and ethanol/water (E/W). The difference in the two constants is too great to be explained entirely by the Born equation (with correction for ion screening) in which the dielectric constant of the solvent system is the pertinent variable. Differential solvation effects appear to be involved. The rate constants, k/sub 2/, for the reaction of the cation with the p-dinitrobenzene anion, p-DNB/sup -/ (p-DNB is the electron acceptor for the photogeneration of the maagnesium octaethylporphyrin cation), and k/sub 4/, for the reaction of the complex with p-DNB/sup -/, are 1.1 X 10/sup 9/ and 2.1 X 10/sup 9/ M/sup -1/s/sup -1/ in M/W and 1.3 X 10/sup 9/ and 1.2 X 10/sup 9/ M/sup -1/s/sup -1/ in E/W. Although the rate constants are within an order of magnitude of being diffusion controlled, the fact that k/sub 4/less than or equalmore » tok/sub 2/ is indicative of some degree of activation control.« less