X-ray crystallography unequivocally showed that protection of the free hydroxyl group of 3,5-O-benzyl-idene-d-lyxono-1,4-lactone with diphenyl-diazo-methane proceeded with retention of configuration to give the title compound, C(25)H(22)O(5). The crystal structure consists of layers of inter-locked mol-ecules lying parallel to the a axis.