This study introduces the first detailed reaction mechanism for coal tar decomposition at moderate temperatures under nonreactive atmospheres; validates the mechanism with accurate interpretations of laboratory tests with diverse coal types; and develops a two-step global reaction scheme to mimic the predictions from the full mechanism in process design applications. Homogeneous tar decomposition reflects two conversion channels: (i) continuous elimination of heteroatoms as noncondensables, which transforms primary tar into polynuclear aromatic hydrocarbons (PAH); and (ii) disintegration of tar monomers with attached hydrogen sources into oils and additional noncondensables. The two conversion channels are coupled by a requirement that monomers disintegrate into oils only if they still have attached peripheral groups to transfer hydrogen into the oils, since the elimination of peripheral groups is a major pathway to eliminate oxygen and hydrogen from primary tar. The aromatic nuclei, labile bridges, char links, and peripheral groups in FLASHCHAIN comprise a suitable basis set of structural components to describe tar decomposition, without modification. Five of the six proposed reactions were also transferred from FLASHCHAIN without modification. Only the channel for oil production is new and distinctive in tar decomposition. All stoichiometric coefficients and all but two activation energies in the proposed reaction mechanism can be evaluated from FLASHCHAIN (R)'s submodel for coal constitution, which requires only the proximate and ultimate analyses for the parent coal. The analysis accurately predicts the reduction in the tar mass, oil yield, and enhancements to the yields of noncondensables for any coal type, given the coal properties and operating conditions. (C) 2016 Elsevier Ltd. All rights reserved.