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• δ2H and δ18O of atmospheric vapour were determined using an evaporation pan method. • Estimates were compared with Fourier Transform Infrared Spectrometer measurements. • Precipitation-equilibrium approach only applies close to the time of precipitation. For a period of 16 months in Sydney, Australia, the variations of 2H/1H and 18O/16O in atmospheric vapour (δ 2H A and δ 18O A) were estimated using an evaporation pan method as well as using the isotopic precipitation-equilibrium approach. These calculations were then compared with δ2H A values measured at 10 m above ground surface using a Fourier Transform Infrared Spectrometer (FTIR). As pan isotopic composition was available on a weekly time scale, the evaporation rates were measured daily, and the atmospheric variables were available hourly, the weekly time scale was used to calculate the arithmetic averages of the atmospheric variables that were used in the estimation of the pan-derived δ 2H A. Good agreement (r = 0.7, P-value = 0.00) was found between the pan-derived and the FTIR measured δ 2H A for weekly intervals, although individual differences ranged from −25.0 to 20.4‰, with the absolute difference averaging 8.0‰. A sensitivity analysis showed that the determination of δ 2H A is most sensitive to air temperature, relative humidity and the isotopic composition of the pan water. While the precipitation-equilibrium approach only appears to be representative of atmospheric conditions close to times of precipitation events, the pan-derived isotopic composition of atmospheric vapour was found to be closer to the FTIR averages over longer periods including intervals with no precipitation. Overall, this means that the pan method is far more effective for uninterrupted estimation of δ 2H A and δ 18O A of atmospheric water vapour, as required for water budget studies, than the precipitation-equilibrium method, and it is more cost effective and robust than continuous measurement. [ABSTRACT FROM AUTHOR]