Oleic acid (OA) and formic acid (FA) react to formoxystearic acid (FSA), in the presence of acidic catalysts.This reaction is of interest as OA can be obtained from renewable resources and FSA is a potential precursor for biopolymers.We have recently studied the liquid−liquid equilibrium (LLE) in the uncatalyzed nonreacting system OA + FA + FSA. Thiswork is extended here to the reactive system, in which a reactive liquid−liquid equilibrium (RLLE) exists. The reactionequilibrium is studied by quantitative NMR spectroscopy at temperatures between 333 and 353 K in homogeneous OA-richmixtures. This information is combined with that on the LLE to determine the RLLE. The physicochemical equilibria aremodeled in a thermodynamically consistent way using the associating lattice cluster theory (ALCT). Furthermore, data on thechemical equilibrium of the formation of the OA-isomer elaidic acid (EA) is supplied. Also the decomposition of FA and theformation of estolides (EL), which are oligomers of OA and EA, was observed and quantified.