Abstract A method of calculating the potential temperature using the pressure integral of the temperature derivative of specific volume is described. The estimation of the uncertainty due to errors in the thermal expansion and specific heat data is straightforward. A comparison is made with values found by Bryden ( Deep-Sea Research, 20, 401–408, 1973) from integration of the adiabatic gradient, the same expressions for the temperature derivative and the specific heat being used in both cases. The results agree within the fitting error in Bryden's polynomial for potential temperature at 35% salinity by Bryden's estimate of the uncertainty in his values is too large. From this analysis, the uncertainty in values of potential temperature in the oceanographic range is estimated to be less than 12 m°C.