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We here report the first molecular characterization f an a-xylosidase (XylS) from an Archaeon. Sulfolobus solfataricus is able to grow at temperatures higher than 80°C, on several carbohydrates at acidic pH. The isolated xy S gene encodes a monomeric enzyme homologous to a-glucosidases, a-xylosidases, glucoamylases and sucrase-isomaltases of the glycosyl hydrolase family 31. XylS belongs to a cluster of four genes in the S. solfataricus genome, including a b-glycosidase, an hypothetical membrane protein homologous to the major facilitator superfamily of transporters and an open reading frame of unknown function. The a-xylosidase was overexpressed in Escherichia coli showing optimal activity at 90°C and a half-life at this temperature of 38 h. The purified enzyme follows a retaining mechanism of substrate hydrolysis, showing high hydrolytic activity on the disaccharide isoprimeverose and catalyzing the release of xylose from xyloglucan oligosaccharides. Synergy is observed in the concerted in vitro hydrolysis of xyloglucan oligosaccharides by the a-xylosidase and the b-glycosidase fro