Prey-predator collagen enrichment values for carbon and nitrogen isotopic compositions are investigated. New enrichment values are given for the well-monitored ecosystem of Bialowieza primeval forest (Poland) for lynx and wolf. The impact of using different approximations in calculating such enrichment values is discussed. Several case studies of ancient vertebrate communities from Upper Palaeolithic sites in southwestern France are presented to check whether the enrichment values estimated for these past ecosystems are consistent with those measured in well-monitored modern ecosystems. The use of ranges of values rather than average ones is recommended, tentatively 0 to 2‰ for δ13C and 3 to 5‰ for δ15N. Copyright © 2003 John Wiley & Sons, Ltd.