Abstract This study describes the influence of neutral or slightly basic clay-equilibrated groundwater on the forward dissolution rate of nuclear borosilicate glasses. The forward rate measured for the SON68 glass, a 26-oxide glass of nuclear interest, is about five times higher in clay-equilibrated groundwater than in deionized water. The corresponding apparent activation energy, measured between 30°C and 90°C, is similar for both media, indicating that glass dissolution is likely controlled by the hydrolysis of Si–O bonds in both solutions. The specific effect of clay-equilibrated groundwater appears to be related to a surface phenomenon; aluminum–oxygen and zirconium–oxygen bonds appear to be less impacted by clay-equilibrated groundwater than silicon–oxygen bonds. More particularly, this work shows that the influence of clay-equilibrated groundwater on the forward dissolution rate is due to its ionic strength with cations predominating over anions, although not all tested cations have the same effect. The effect is due mainly to calcium ions, although the concentration of sodium ions in the clay-equilibrated water tested was four times higher than that of calcium ions. The specific influence of alkali metal (Li+, Na+, K+, Rb+, Cs+) and alkaline earth (Mg2+, Ca2+, Sr2+, Ba2+) ions shows that all cations do not have the same effect on the forward glass dissolution rate and that the rate increases with the mole fraction of surface metal sites. The presence of calcium in the glass does not prevent this particular effect.