[en] An isotope effect was observed in pH-stat experiments when comparing the leaching rates of sodium from sodium trisilicate glass in deuterated and protonated water. The magnitude of the effect, defined as the ratio of leaching rates in D₂0 to those in H₂0, was 0.70 + - 0.07. The isotope effect persisted at this magnitude both in the initial stages of sodium ion release, usually considered to be controlled by ion interdiffusion, and in later stages, usually attributed to dissolution of the silicate matrix. No temperature dependence was apparent. While diffusion processes quite probably influence initial leaching rates, they are unlikely to give rise to the observed isotope effect. Rather, results can be explained using transition state theory involving a proton transfer step in the hydrolysis of the silicate matrix. Matrix hydrolysis is proposed as an important precursor to alkali release, initially as well as in later stages of the reaction. (author)