Complex formation of molybdate and of tungstate with lactate have been investigated in the pH_c range 2–7.5 by potentiometric and enthalpimetric titrations at 25 °C in 1.0 mol dm^–3 NaCl. The potentiometric data were treated with the computer program SUPERQUAD taking into account the side-reactions of molybdate and tungstate with hydrogen ions. For both systems the ‘best’ reaction model comprises only one major complex and a number of minor complexes. Thermodynamic quantities have been determined for each of the major complexes. For the molybdenum(VI) complex, [MoO₂(C₃H₄O₃)₂]^2–, the values for the formation constant and enthalpy change are log β₁₂₂= 15.71 and ΔH^⊖=–72 kJ mol^–1 respectively and for the tungsten(VI) complex, [WO₂(C₃H₄O₃)₂]^2–, the values are log β₁₂₂= 17.47 and ΔH^⊖=–80 kJ mol^–1. The equilibrium constant and enthalpy change for the protonation of lactate have also been determined, log K= 3.60 and ΔH^⊖=–1.7 kJ mol^–1. The energetics of complexation is discussed in terms of the thermodynamic quantities.