Heat capacities of (Zr(1-y)M(y))O-0.17 and (Zr(1-y)M(y))O-0.28 (where M is Nb or Sn, y = 0-0.07) having alpha"-ZrOapproximate to 1/6 -type and alpha"-ZrOx-type crystal structures, respectively, were measured from 325 to 905 K using an adiabatic scanning calorimeter. A heat capacity anomaly at higher temperatures due to an order-disorder rearrangement of interstitial oxygen atoms in zirconium based alloys was observed for all samples, after the heat capacity anomaly at lower temperatures had been attributed to a non-equilibrium phenomenon. The transition temperature, transition enthalpy and entropy changes due to the order-disorder transition decreased with increasing doped niobium or tin content, indicating that arrangement of oxygen atoms in the lower temperature phase may be partially disordered by substituting niobium or tin for zirconium. The entropy change due to the order-disorder transition for (Zr(1-y)M(y))O-0.17 and (Zr(1-y)M(y))O-0.28 solid solutions was compared with the theoretical value. The solubility limits of (Zr(1-y)M(y))O-0.17 and (Zr(1-y)M(y))O-0.28 were determined from the variation of lattice constants, transition temperature, transition enthalpy and entropy changes against niobium or tin content.