The heat capacities of La2O2CO3 and Nd2O2CO3 were determined by adiabatic shield calorimetry from T = (12 to 300) K and T = (12 to 930) K, respectively. For the dineodymium dioxide carbonate a Schottky-type heat capacity with maximum near T = 60 K was observed. The transition is due to splitting of the I-4(9/2) ground term in the crystal field of the Nd3+. The vibrational contributions to the heat capacities of La2O2CO3 and Nd2O2CO3 are discussed in terms of a modified Kieffer model. Although the heat capacity can be modelled reasonably well, deviations, especially at low temperatures, give rise to significant errors in entropy. Thermodynamic properties at selected temperatures have been evaluated, and the values of the molar heat capacities C-p,C-m, enthalpy increments Delta(0)(T)H(m), and entropies Delta(0)(T)S(m) for La2O2CO3 at T = 298.15 K and for Nd2O2CO3 at T = (298.15 and 900) K are (R = 8.3145 J.K-1.mol(-1)): [GRAPHICS]