The complexation reactions between 4-(2-pyridylazo)resorcinol (PAR) and Al3+, Fe3+, Zn2+, and Cd2+ ions at 25 degrees C and 0.1 M ionic strength are performed spectrophotometrically. A critical comparison of the various PCA methods on the absorbance matrix data concerning the number of light-absorbing species has been made. Knowing the number of light-absorbing species is a critical step for subsequent quantitative and qualitative solution equilibria studies. Therefore, the nine selected index functions for the prediction of the number of light-absorbing components which contribute to a set of spectra are critically tested using the algorithms implemented in INDICES software. Behind the number of various complexes formed the stability constants of species ML and MLH, type log beta(110) and log beta(111), for the system of PAR (ligand L) with Zn2+ and Cd2+ ions and the stability constants of species MLH and ML2H, type log beta(111) and log beta(121), for the system of PAR (ligand L) with Al3+ and Fe3+ ions at 25 degrees C and 0.1 M ionic strength, and various mole ratios are estimated by the SQUAD and EQUISPEC programs.