Protolytic properties of porphyrin complexes with Mg, analogs of the natural porphyrin chlorophyll, were investigated by the UV spectroscopy method with simultaneous potentiometric determination of pH of the medium with the aim to detect metalloporphyrin forms arising in a solution with changing its acidity. To consider interconversions in solutions, MCR-ALS soft modelling based on the chemometric analysis of spectrometric data was applied.