The apparent equilibrium constants and enthalpies of complexation of Nd3+, Sm3+, Eu3+, and Gd3+ by xylitol in aqueous solutions containing NaNO3 at an ionic strength of 2.0 mol-kg−1 have been determined by microcalorimetry at 25°C. Since nitrate anion weakly complexes the lanthanide cations, these values are analyzed in terms of competition between xylitol and NO−3 The method leads to the apparent equilibrium constants and enthalpies of complexation of the lanthanide cations by NO−3 at this particular ionic strength. Despite the difficulties encountered in characterizing rather weak associations, the results are, whenever comparison is possible, in good agreement with those obtained by direct microcalorimetry. The advantage of this competition method is that it can be used when the enthalpic effects are too weak and insufficiently concentration dependent for direct microcalorimetric determination. In the present case, it allows us to thermodynamically characterize the formation of SmNO2+3 and EuNO2+3, processes we have not been able to study directly.