Heat-treated initial bentonite and its Al and Fe(III) cation-substituted forms which were exposed to thermal activation at 105, 200, 400, 600, 8000С within 6 hours have been produced. Their sorption capacity by methylene blue from solutions (concentration of methylene blue in solutions made up 0.02 mol/l) was studied in dynamic conditions. Results of experimental researches show that at 105 and 2050С sorptive capacity of both natural and modified samples practically does not change, but by increasing the temperature up to 8000С significant reduction of sorptive capacity is observed and this value is found to be around 30% at 4000С. Performed investigations show that selection of optimum sorbent graining is of significant value in sorption processes of treating waste waters, therefore dependence of sorption of cationic dyes on sizing fractions of sorbents was investigated in dynamic experiments.