G3MP2B3 calculations were used to investigate the mechanism of the phosgenation reactions of 4,4'-methylenedianiline (MDA) resulted in the formation of methylene diphenyl diisocyanate (MDI). Due to o-dichlorobenzene solvation the reaction barriers are dramatically reduced compared to the gas phase reaction mechanism. The mechanisms of the 'phosgenations first' and 'stepwise phosgenations' are energetically compared. Standard enthalpy of formation for MDI (14.8 +/- 5.2 kJ/mol) and the group additivity increments for OCN and NHCOCl groups linked to phenyl ring are proposed (Delta H-f,i(0)(-NCO) = -61.2 kJ/mol and Delta H-f,i(0)(-NHCOCl) = -195.0 kJ/mol). A thermodynamic stabile charge separated amine hydrochloride intermediate can mask the amine toward phosgene. (C) 2018 Elsevier B.V. All rights reserved.