Macrobenthic infaunal sampling campaigns that aim at unpreserved "fresh" animals for later biochemical analysis follow one of two strategies: either samples are sieved and sorted on board and individual specimens are frozen, or grab/corer samples are frozen completely and animals are separated from sediments later in the laboratory. This study demonstrates that deep freezing of complete grab samples causes a significant bias in stable isotope ratios of nitrogen (15N/14N or δ15N vs. AIR) and carbon (13C/12C or δ13C vs. PDB) as well as in the C/N ratio of nine infaunal species from the German Bight. On average, molar C/N ratio increases by 0.93, δ13C decreases by 1.87‰, and δ15N decreases by 1.01‰. Mechanical cell destruction and subsequent loss of cytosol as well as metabolic degradation by free enzymes and by microorganisms are discussed as major causes for the observed effects. We recommend to abstain from using bulk frozen grab samples for the analysis of C/N ratio or stable isotope ratios. © 2007 Elsevier B.V. All rights reserved.