The mechanism of the reaction that produces maleic and succinic anhydrides from carbon monoxide and acetylene was studied in the catalytic PdBr 2 -LiBr-CH 3 CN system. The in situ study of reactivities of possible organic intermediates, the kinetic isotope effect study (for succinic and maleic anhydrides k H /k D was estimated to be 1.05+/-0.05 and 0.9+/-0.1, respectively), the study of isotope exchange, and the oxygen effect on the process direction, revealed that maleic anhydride is most likely a key intermediate of the succinic anhydride formation. Maleic anhydride undergoes transformations through the mediation of a palladium hydride complex. This complex was detected in the catalytic solution at -40 o C with H 1 NMR (δ = -1.88 ppm, ν 1 / 2 = 70 Hz).