Calcium(II) and magnesium(II) complexes of partially deprotonated H4SiO4 have been investigated at 25 °C in solutions of constant ClO4– molarity (1 M) by computer-controlled coulometric titration using a hydrogen electrode. The experimental data could be explained assuming the following equilibria. M2++ H3SiO4–⇆ MH3SiO4+; K1=[MH3SiO4+]/[M2+][H3SiO4–] and M2++ 2H3SiO4–⇆ M(H3SiO4)2; β2=[M(H3SiO4)2]/[M2+][H3SiO4–]2 or M2++ H2SiO42–⇆ MH2SiO4; χ1=[MH2SiO4]/[M2+][H2SiO42–] For the individual equilibrium constants, the following values were obtained: Ca2+: log K1= 0·39 (±0·03); log β2= 2·89 (±0·07)(log χ1= 3·09) Mg2+: log K1= 0·64 (±0.06); log β2= 3·82 (±0.08)(log χ1= 4·17)