The protonation reaction rate of the triplet state of the dye Safranine-T has been used to determine the local proton concentration on the microdomains created by surfactants, polyelectrolytes, and copolymers of charged and neutral monomers with different charge densities, when dissolved in aqueous solutions. The proton concentration effect is larger for micelles than for polyelectrolytes, and it decreases with the amount of neutral copolymerized neutral monomers. The proton concentrations determined in this way are consistent with those calculated from the electrical potentials in the Stern layer.