This paper presents high-resolution records of 13C/12C ratios of organic matter from two loess sequences in northwestern Europe. Our analysis is the first attempt to use organic matter δ13C as a record of the response of vegetation to climatic variations in an area where climatic changes were not strong enough to induce a radical change in vegetation cover. Over the last climatic cycle, the vegetation of the Rhine Valley showed a strong predominance of C3 plants. Thus, the small δ13C variations, with an amplitude of only 1.5‰ to 3‰, are interpreted as corresponding to fluctuations in water supply and atmospheric CO2 concentration variations rather than to the ratio of C4 to C3 vegetation. Furthermore, loess sequences accumulated at high rates and allow high correlation with climatic proxy data, like the Greenland Ice Sheet Project 2 (GISP2) δ18O and the variations in CO2 concentration recorded in the Vostok ice core. The δ13C constitutes a reliable and complementary proxy to study small climatic stresses endured by vegetation during the Weichselian in northwestern Europe. Moreover, by using absolute age control and correlations between global (Vostok-CO2) or semi-global (GISP2-δ18O) climate effects, δ13C values of organic matter in loess sequences offer a new tool to establish a refined chronology in continental sequences.