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Values of delta C-13 are frequently reported with radiocarbon dates from organic materials. In C-3 plants delta C-13 values have been linked to changes in water use efficiency as a response to arid conditions. By calculating C-13 discrimination (Delta C-13) from C-13 isotopic composition (delta C-13), archaeologists can gain potentially valuable inference into past climate conditions. Values of Delta C-13 reflect the process of discrimination against heavier C-13 isotopes of carbon by comparing the delta C-13 of samples to that of the atmosphere, and can be calculated when records of atmospheric delta(CO2)-C-13 are available. The present study examines a 1300 year history of radiocarbon-derived 6,13C from the Lower Alentejo of Portugal using charcoal recovered from excavations of a series of medieval habitation sites in the study area. To calculate Delta C-13, the posterior means generated from Bayesian change-point analysis of delta(CO2)-C-13 records were used. Archaeological data were then compared to contemporary ecological studies of Delta C-13 of the same taxa against instrumental records of climate. Values of Delta C-13 fell within mean ranges for the taxa through a period of population growth between the 7th and 10th centuries AD. During the height of the Medieval Warm Period in the 11th century AD Delta C-13 values frequently fell to low levels associated with arid conditions. At this time environmental degradation and erosion were documented. Values of Delta C-13 increased for a brief period in the early 12th century AD before the rural Lower Alentejo was largely abandoned for nearly two centuries. Another period of aridity occurred in the 16th and 17th centuries AD. Radiocarbon-derived Delta C-13 is a potentially useful paleoclimate proxy for archaeologists provided that results can be paired with observed Delta C-13 variation in studies that pair these data with instrumental climate records. (C) 2012 Elsevier Ltd. All rights reserved.