Catalytically efficient beta-D-xylosidase from Selenomonas ruminantium ( SXA) exhibits pK(a)s 5 and 7 ( assigned to catalytic base, D14, and catalytic acid, E186) for k(cat)/ Km with substrates 1,4-beta-D-xylobiose ( X2) and 1,4-beta-D-xylotriose ( X3). Catalytically inactive, dianionic SXA ( D14(-)E186(-)) has threefold lower affinity than catalytically active, monoanionic SXA ( D14(-)E186(H)) for X2 and X3, whereas D14(-)E186(-) has twofold higher affinity than D14-E186H for 4-nitrophenyl-beta-D- xylopyranoside ( 4NPX), and D14(-)E186(-) has no affinity for 4-nitrophenyl-alpha-L-arabinofuranoside. Anomeric isomers, alpha-D-xylose and beta-D-xylose, have similar affinity for SXA. 4-Nitrophenol competitively inhibits SXA-catalyzed hydrolysis of 4NPX. SXA steady-state kinetic parameters account for complete progress curves of SXA-catalyzed hydrolysis reactions.