Very little is known about the effect of overgrazing on carbon loss from soil in semi-arid savannas and woodlands of South America. Soil carbon parameters were measured in a 10,000ha restoration project in the western Chaco of Argentina (24 o 43'S and 63 o 17'W). Three situations were compared: highly restored (HRS), moderately restored (MRS) and highly degraded (HDS). Soil and litter samples were recovered in the dry and wet seasons. SOC and CO 2 -C values decreased from the HRS (7.0kgm - 2 and 130gm - 2 ) to the HDS (1.5kgm - 2 and 46gm - 2 ) whereas the C mineralization rate increased toward the less restored sites (0.96-2.29). Surface-litter C was similar in both sites under restoration (260 and 229gm - 2 ), being non-existent at the HDS. Leaves from woody species dominated surface-litter in the HRS, whereas grass material was predominant in the MRS. During the wet season, the SOC decreased, whereas both CO 2 -C and C mineralization rate increased. The magnitude of the between-season differences was highest at the HDS (62% in SOC, 55% in CO 2 , and 80% in C mineralization rate). We estimated that C loss since introduction of cattle into the forest was 58Mgha - 1 , reaching a total of 2x10 1 5 g at for the entire Chaco. These values are higher than those caused by the conversion of savannas and other ecosystems into agriculture or cultivated pastures. The amount of C fixed in the highly restored site (275gha - 1 per year) indicates that the Chaco soils have a significant potential as atmospheric carbon sinks.