The isotopic ratios 13C 12C and 18O 16O of cellulose from tamarix trees which were used by the Roman army as a groundwork of the siege-rampart of Masada (ad 70-73) were compared with ratios measured in present-day tamarix trees growing in the Masada region and in central Israel. The ancient tamarix cellulose is depleted in both 13C and 18O compared to cellulose from trees growing in the Masada region today. Similar trends were observed on comparing modern tamarix trees growing in the Negev Desert with those growing in the temperate climate of central Israel. Considering the factors that can contribute to the observed changes in isotopic composition, we conclude that the ancient trees enjoyed less arid environmental conditions during their growth compared to contemporary trees in this desert region. This report demonstrates the potential in using combined 18O and 13C analyses of archeological plant material as independent indication of regional climatic change in desert areas (where conventional isotopic analyses, such as in tree rings, are impractical).