Nicotinic acid (also known as pyridine-3-carboxylic acid) is widely used in food, pharmaceutical and biochemical industries. The extraction of nicotinic acid by tri-noctylphosphine oxide (TOPO) with kerosene (diluent) is studied. The equilibrium experiments are carried out to investigate the effects of diluent, extractant (TOPO) composition and initial acid concentration on extraction efficiency. The extraction efficiency is found to increase with increasing TOPO composition concentration (0.10 to 0.60 mol.L -1) and found to decrease with increasing initial acid concentration (0.02 to 0.12 mol.L -1). The number of TOPO molecules in the acid:TOPO complex is estimated through proposed mathematical model using graphical methods as well as though an optimization route using the differential evolution algorithm and found to be 1:1 complexes of TOPO and acid with loading ratio of less than 0.5. The extraction equilibrium constants are also determined. Equilibrium extraction constant (K E1) is found to be 3.27 L.mol -1 in the complete range of acid concentrations and extractant (TOPO) concentrations.