A problem in the use of reactive extraction to recover carboxylic acids from bioreactor products is the toxicity of extractants and diluents used. Selection of an extractant and diluent for reactive extraction should be on the basis of minimal toxicity and maximum capacity. Pyridine carboxylic acids and their derivatives are attracting considerable attention for their presence in many natural products. 2-pyridine carboxylic acid, known as picolinic acid and 3-pyridine carboxylic acid, known as nicotinic acid are widely used in the pharmaceutical industries. Present paper focuses on reactive extraction of picolinic acid and nicotinic acid using tri-n-butyl phosphate (tri-n-butyl phosphate) in natural non-toxic soybean oil. Comparative results were presented in terms of distribution coefficients (0.01-0.374 for picolinic acid and 0.0073-0.0752 for nicotinic acid), loading ratio (<0.5 for both acids), degree of extraction (0.9-27.22% for picolinic acid and 0.72-6.99% for nicotinic acid), and equilibrium complexation constants. Three different models: relative basicity, mass action law and Langmuir models were used to represent the reactive extraction equilibrium for picolinic acid/nicotinic acid - tri-n-butyl phosphate - diluent. The best suitability of model was compared using root mean square error (RMSE), reduced chi-square value (chi(2)) and percent relative error. (C) 2013 Elsevier B.V. All rights reserved.