Continuous time series of δD and δ18O in atmospheric water vapour and precipitation at Heidelberg are presented for the years 1981 to 1988. It is often not possible to derive the isotopic composition of the water vapour from the isotopic composition of precipitation for individual months. However, a close relationship does exist between the isotopic composition of the vapour and the precipitation on a timescale of a few years. The correlation of isotopic composition with temperature is much better for the vapour data than for the precipitation data. The Δδ/ΔT relationship of the seasonal variations is itself a function of temperature, being much lower during warmer periods than during cold periods. Especially during summer months, the admixture of water vapour originating from plant transpiration plays an important role in biasing the Δδ/ΔT relationship towards lower values. This effect complicates the calculation of palaeotemperatures using isotopic data.