The specific purpose of this study was to compare three different collagen extraction methods commonly used in isotope laboratories conducting dietary studies. We evaluated their resultant differences in d13C and d15N, collagen quality and collagen yield. Our study was based on well-preserved skeletal material from the medieval period in Denmark. Our study shows that there is a systematic significant difference in the yield and the d13C values between the three methods. Using the method of DeNiro and Epstein [DeNiro, M.J., Epstein, S., 1981. Influence of diet on the distribution of nitrogen isotopes in animals. Geochimica et Cosmochimica Acta 45, 341e351] with NaOH as cleaning agent, will, according to our study, give d13C values that are on average 0.32& more positive than using the ultra-filtration method [Brown, T.A., Nelson, D.E., Vogel, J.S., Southon, J.R., 1988. Improved collagen extraction by modified Longin method. Radiocarbon 30 (2), 171e177, modified in Richards, M.P., Hedges R.E.M., 1999. Stable isotope evidence for similarities in the types of marine foods used by late Mesolithic humans at sites along the Atlantic coast of Europe. Journal of Archaeological Science 26, 717e722]. The third method, which is a modified version of the second method, excluded the ultra-filtration step. This method seems to give d13C values that lie in between the other methods. Our study did not show any significant difference in d15N values. Although the differences between the methods are very small, we conclude that the use of stable isotope analysis in food determination studies requires adherence to routine methods for preparing and measuring samples. 2007 Elsevier Ltd. All rights reserved.