The protonation of tetraphenylporphyrin followed by spectrophotometric detection of the resulting dihydrogen tetraphenylporphyrin dication has been evaluated as a means to determine micromolar concentrations of strong acids in chloroform. The equilibrium constants for the process are large (>1010 for benzenesulfonic and methanesulfonic acids), but not large enough to ensure stoichiometric conversion to the dication unless only order of magnitude precision is required. The incorporation of a correction term in the stoichiometric approximation, based on a dilution of the analytical sample, yields an acceptable result for these two acids.