Adsorption capacity of chitosan towards toxic p-nitrotoluenosulfonic acid (PNTS) was investigated in this study. An adsorption isotherm was determined at 293 K. The character of the process was specified. On the basis of calorimetric measurements the thermal power of the process was determined. The investigations revealed that chitosan was a good PNTS adsorbent, the adsorption of this compound had a character of chemisorption and took place on the adsorbent surface.