The heat capacity of ammonium hexachloroosmate (NH4)(2)OsCl6 was measured at temperatures from T = 6 K to 325 K by adiabatic calorimetry. Two small anomalous regions were observed in the heat-capacity against temperature plot. The lower one, from 80 less than or equal to (T/K) less than or equal to 140, had an excess molar enthalpy of (21.19 +/- 0.04).R.K and an excess molar entropy of (0.198 +/- 0.006).R. The upper one, from 200 less than or equal to (T/K) less than or equal to 300, had an excess molar enthalpy of (19.51 +/- 0.04).R.K and an excess molar entropy of (0.076 +/- 0.006).R. Smoothed values of the standard molar thermodynamic quantities for pure (NH4)(2)OsCl6 are tabulated up to T = 350 K.