Heat capacities of ZrOx(x = 0-0.31) were measured from 325 to 905 K using an adiabatic scanning calorimeter, Two kinds of heat capacity anomalies were observed for all samples except pure zirconium metal. A lambda-type heat capacity anomaly was observed in the temperature range from 600 to 750 K and was assigned to an order-disorder rearrangement of the interstitial oxygen atoms in zirconium host lattices. Another anomaly around 400-500 K depended on heating conditions, indicating that this anomaly is due to a non-equilibrium phenomenon. Due to the imperfect ordering of the sample during the cooling process, a temporary ordering and a subsequent recovery into an equilibrium state occur during the heating process at a finite rate, making a broad heat capacity anomaly. This behavior is discussed with reference to the results of the dependences of the cooling rate of the sample and the heating rate of the calorimeter on the heat capacity anomaly.