The dissolution rate of CeO 2 and Co 3 O 4 in 7 M nitric acid is investigated to elucidate whether microwaves have a non-thermal effect upon this type of dissolution reaction. CeO 2 and Co 3 O 4 were selected as model oxides because they typically represent refractory mineral materials. Nitric acid was chosen as it is a preferred leaching medium used in the nuclear industry. From preliminary experiments performed with conventional heating, the activation energy of the dissolution process was found to be 102 ± 9 kJ mol - 1 and 110 ± 12 kJ mol - 1 for CeO 2 and Co 3 O 4 , respectively. The microwave-assisted dissolution experiments were performed with a Teflon bomb under strict control of temperature and pressure. The results obtained show that the action of the microwaves is purely thermal in nature and thus does not provide any detectable specific activation. However, the use of microwaves is highly interesting because it provides a convenient way of transferring energy to a given system and, chiefly, gives rise to an apparent acceleration of the chemical reactions as a result of the superheating phenomenon.