X-ray crystallography unequivocally confirmed the stereochemistry of the 2-C-methyl group in the title mol-ecule, C(10)H(16)O(5), in which the 1,5-lactone ring exists in a boat conformation. The absolute stereochemistry was determined by the use of d-ribose in the synthesis. The crystal exists as O-H⋯O hydrogen bonded chains of mol-ecules running parallel to the a axis with each mol-ecule acting as a donor and acceptor for one hydrogen bond.