Surface complex formation of K+, NO3-, SO42-, Ca2+, F-, Co2+, and Cr3+ ions was determined on the surface of silica gel. Experimental data obtained by acid-base titration of suspensions were interpreted in terms of the triple-layer model. The value of the deprotonation constant of surface OH could be determined precisely but the protonation constant was rather uncertain. The logarithms of ion pair formation constants for K+, NO3-, Ca2+, and SO42- adsorbed in the beta -plane are log K-ipM,K-X similar to 0, therefore these species can be considered inert ions in the investigated pH range. F-, Co2+, and Cr3+ ions were found to be strongly sorbed in the o-plane. In order to provide a good fit and to obtain parameters independent of their initial values, all possible equilibrium must be accounted for in the models.