Pyridine-3-carboxylic acid (also known as nicotinic acid) is widely used in food, pharmaceutical, and biochemical industries. The production of acid can be intensified by enzymatic conversion of 3-cyanopyridine or biosynthesis and by reactive extraction used as a separation step from a dilute fermentation broth. The extraction of pyridine-3-carboxylic acid by 1-dioctylphosphoryloctane, also known as tri-n-octylphosphine oxide (TOPO), with five different diluents (n-octane, methylbenzene, 4-methyl-2-pentanone (MIBK), decan-1-ol, and dichloromethane) 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.m(-3), and found to decrease with increasing initial acid concentration, (0.02 to 0.12) mol.m(-3). The number of TOPO molecules in the acid:TOPO complex is estimated through a proposed mathematical model using graphical methods as well as through ail optimization route-using the differential evolution algorithm and found to be 1:1 complexes in most of the cases. The extraction equilibrium constants are also determined. In all the tested diluents, methylbenzene containing the benzene ring in the structure is the best solvating agents for acid-TOPO complexation giving K(EI) = 5.04,