At the German wintering base Neumayer, an intensive glacio-meteorological program was carried out during the last two decades. A complete meteorological data set and data from surface snow samples, snow pits, firn cores, and weekly accumulation measurements from a stake array are available. We first investigated the attenuation of the seasonal d18O signal due to water vapour diffusion in the snow pack. A comparison of surface snow samples and firn cores of different age shows that only one third of the seasonal d18O signal of the surface snow samples remains in the cores after the first year. No further significant change in the amplitude of the seasonal d18O signal is found later. Changes in the seasonal distribution of accumulation can lead to a bias in ice core properties. This is studied on a short time scale, using the available high-time resolution data of accumulation, stable isotope ratios, and air temperature. Mean annual d18O values from firn cores are not well correlated to annual mean air temperatures. However, the correlation is improved considerably by weighting the annual mean air temperature by accumulation. At Neumayer, mainly the cyclonic activity in late winter/early spring determines whether and how the core data are biased.