Hearths are important archaeological features, serving to infer past practices related to hominin subsistence and social behaviors. The identification of hearths is not always straightforward due to post-depositional processes. In karstic cave environments in particular it is not always easy to distinguish, in the field or in the laboratory, between geogenic and anthropogenic cemented features because both are composed of calcite. Here we present a novel study in which bulk oxygen and carbon isotopic analyses were used in order to unequivocally identify anthropogenic calcite derived from hearths in the Neanderthal site of Amud Cave (Israel). We demonstrate that the isotopic composition of wood ash is distinctively different from that of geogenic calcite. The linear relationship observed between carbon and oxygen isotopic compositions of the various sediment types in Amud Cave is interpreted as a mixing line between wood ash and geogenic calcite, thus enhancing our understanding of the origins of sedimentological variability at the site and showing that not all of the cemented sediments in Amud Cave should be regarded as hearths. Our study suggests that the relative abundance of validated hearths in the various areas of the cave may be associated with Neanderthal use of space in Amud Cave. © 2008 Wiley Periodicals, Inc.