Various seed galactomannans differ from each other in G:M ratios, molecular weight and fine structure, w.r.t. the distribution of galactose side groups on the main chain. In the present investigation, galactomannan samples of fenugreek, guar and locust bean having different G:M ratios of 1:1, 1:2 and 1:4 respectively, were exposed to different doses of gamma radiation. Dispersions of the resulting products were prepared and their viscosities were measured. A considerable fall in their viscosities was noted. The lower viscosity is obtained due to the production of lower molecular weight products, indicative of depolymerization. It was observed that galactomannan having the highest galactose : mannose (G:M) ratio of 1:1 resisted depolymerization the most in comparison to the galactomannan possessing the lowest G:M ratio of 1:4.