A solution has been developed to the problem of pH control in interactive (finite dilution) leach tests on waste forms. To make sure that the pH, which exercises a major influence on the reactivity of the medium, is controlled by extraction of soluble components of the waste form (alkalis, silicate, borate, etc.) into the leachant and not by extraneous factors originating in the testing system (CO2, fluoride, etc.), the complete ionic balance in the leachate is determined. A technique for carrying out reliable pH measurements on small samples of unbuffered leachants by means of a glass electrode in a flow cell has been developed. The achievement of satisfactory cation-anion balance in leach solutions using chemical and electrochemical techniques has made it possible to identify in every case the major species that determine the pH and to distinguish between interactive and constant-medium test configurations. Testing systems that minimize CO2 infiltration have been developed.