The periodic acid oxidation of the ‘glycogens’ in sections of ox liver fixed in Carnoy's fluid has been studied quantitatively. It was found that in 0.02 M acetate buffer at pH 5.0 the oxidation was rapid at first but levelled off after I hr. Even after prolonged oxidation (12 days), not more than 23–24% of the total available glycogen was oxidized. However, in the presence of electrolytes (e.g. 0.2.m sodium chloride) the oxidation was much greater. After 2.5 hr, for example, 40% of the available glycogen was oxidized. There was little difference in the velocity of oxidation in sections mounted on glass-slides and free-floating ones, or in free-floating sections of different thicknesses. Mounted and unmounted sections consumed on average 11–17 times more periodate than could be accounted for by the oxidation of their glycogen content. The results are interpreted in terms of a complex formed between periodate ions and the cuter glucosyl residues of the glycogen aggregate. The negative charge on the complex, it is suggested, prevents free periodate ions approaching and oxidizing the inner glycosyl residues.