An in-situ RBS system has been developed to study the dissolution of Pb layers deposited physically on the SiO2 surface of Si(100) crystal into water solutions with different pH values. It is found that Pb atoms are not dissolved into alkaline water, but into acid water, and that the dissolution in the latter case is the zero-th order reaction kinetics and the rate constant is 0.67 x 10(13) atoms cm(-2)s(-1), which corresponds to 1.04 x 10(-11) mol.cm(-2)s(-1). The dissolution mechanism is discussed based on the experimental results.