Although organotin carboxylate compounds are widely used in the polyurethane industry as catalysts, their reactions mechanisms are not fully understood. The urethane formation between isocyanate and alcohol groups in the presence of organotin carboxylate catalysts is simulated with selected compounds using computational methods and the results are compared with experimental findings. The results reveal that organotin carboxylate catalysis of the reaction between these isocyanate and n-butanol in a non-polar medium follow two routes depending on the isocyanate structure. The reaction rate for aliphatic primary isocyanates the concentration of the carboxylate ligand in the catalyst has a significant effect while for aromatic isocyanates the effect is much less. Based on experimental and computational results a mechanism is proposed for organotin carboxylate catalysis of urethane formation for these isocyanates. (c) 2012 Elsevier B.V. All rights reserved.