The enthalpy of cadmium adsorption onto rutile (TiO2) was determined microcalorimetrically. Thermodynamic analysis of the temperature dependence of cadmium adsorption provides an independent estimate of this enthalpy. By introducing a model to account for enthalpic contributions due to OH- co-adsorption, we show that results from both methods are consistent. Adsorption of the cadmium ion as such is enthalpically unfavourable. However, because of concomitant OH-/H+ adsorption, both the overall adsorption and desorption are exothermic processes.