We use femtosecond midinfrared pump-probe spectroscopy to compare the ultrafast dynamics of HDO dissolved in D2O and H2O. For both systems the vibrational energy relaxation proceeds through an intermediate state. The relaxation leads to heating of the sample, which is observed in the transient spectra. In order to obtain the correct anisotropy decay, the ingrowing heating signal is subtracted from the raw data. For the OD vibration this procedure works well. For the OH vibration, however, we find an additional effect that leads to a severe distortion of the anisotropy. We show that this effect can be explained by a slightly faster reorientation of excited molecules during their relaxation as compared to unexcited molecules. We construct a model that includes this effect and is able to reproduce the experimental data. Using this model we show how the distorted anisotropy can be corrected.