Galactomannan samples of different origins (guar gum, tara gum, and locust-bean gums), with mannose-to-galactose (M/G) ratios from 1.7 to 4.8, have been characterized. They exhibited strong differences with respect to their molecular weight and their fine structure (M/G). The different locust-bean-gum samples differed significantly in this respect from each other. All galactomannans were submitted to periodate oxidation. The periodate oxidation of galactomannan samples was characterized by the formic acid liberated. As expected, the formic acid liberated was directly related to the M/G ratios of galactomannan samples. Thus galactomannan periodate oxidation was shown to be a simple and precise analytical method to predict the M/G ratio of galactomannan samples. It was demonstrated that galactomannan periodate oxidation, as a measure of the M/G ratio, can be directly related to the physical functionality of galactomannans in terms of synergistic interactions between the kappa-carrageenan and the galactomannans.