Stoichiometric evaluation of the radical scavenging activity of O-substituted derivatives at the C-2 position of ascorbic acid (AA) was conducted. Their reaction with a stable radical, 1,1-diphenyl-2-picrylhydrazyl (DPPH), under an acidic condition was assessed by the colorimetric method. 2-O-alpha-D-Glucopyranosyl-L-ascorbic acid (AA-2G) and a series of 6-O-acyl-2-O-alpha-D-glucopyranosyl-L-ascorbic acids (6-Acyl-AA-2G) had long-term radical scavenging activity against DPPH. The reaction of AA-2G or 6-Acyl-AA-2G with DPPH was very slow when compared with AA. However, one molecule of these derivatives consumed approximately three molecules of DPPH radicals at the end of the experiment (2 h). In contrast, one molecule of AA scavenged two molecules of DPPH radicals, and the reaction ended in the short time (<10 min). The quantity of radicals quenched by AA-2G and 6-Acyl-AA-2G was superior to that of AA in a long-term reaction.