Activated carbon has been used for the adsorption of nicotinic acid at 30, 40, 50 and 60°C in pure water and in the presence of 0.1 M NaCl solution. Various different equations for the adsorption isotherms have been tried to fit the adsorption data using a computer program, and it is observed that all the isotherms follow a Langmuirian pattern. The thermodynamic parameters have also been reported for both systems. The mechanism of adsorption has been explained on the basis of these parameters where the presence of an electrolyte (NaCl) plays a significant role in the adsorption process.