Previously derived equations relating the specific rotation and chemical composition of 4‐O‐methylghicurono‐xylans and arabino‐4‐O‐methylglucurono‐xylans have been found to be partly incorrect because of the assignment of an erroneous value to the rotatory contribution of the 4‐O‐methyl‐α‐D‐glucuronosyl xylose residue. The correct value was calculated from the known molar rotations of xylobiose and 22‐(4‐O‐methyl‐α‐D‐glucuronosyl) xylotriose. Approximately the same values were obtained from the specific rotations of five carefully purified 4‐O‐methylglucurono‐xylans, containing different proportions of acid side chains, and from the rotations of an aldopentaouronic and an aldohexaouronic acid. A revised equation was used for estimating the specific rotation of several arabino‐4‐O‐methylglucunmo‐xylans from various softwoods. Contrary to the earlier results, a satisfactory agreement was found to exist in most cases between found and calculated values.