The proton binding isotherms of simple and mixed hydroxides were measured by potentiometric titration. The hydroxides were selected so that the metal cations (Al, Ni, Ri, and Ca) were of different polarizing power, expressed as Z/r(2), where Z is the cation charge and r its radius. The results show that the acidity of the simple hydroxides increases as Z/r(2) increases. The basicity follows a reverse order and is not completely congruent with Z/r(2). The experimental PZC was found to increase with basicity. A correlation between basicity (or PZC) and Z/r(2) has been noticed when the latter was normalized to (CN)(0), the coordination number of oxygen in each hydroxide, and it has been shown that for simple and mixed hydroxides the PZC increases as (Z/r(2))/(CN)(0) decreases. Such dependence is interpreted in terms of hydrolysis of surface metal cations and protonation of surface O-2/OH- ions. An empirical equation is proposed which predicts the point of zero charge of many metal (hydr)oxides, confirming the suggested role of the parameter Z/r(2) of the metal cation in controlling the charge at the metal (hydr)oxide interface.