The kinetic isotope effect (KIE) in the decarbonylation of formic acid(FA) assisted with acetic anhydride (AA) has been studied in the temperature interval 18–100°C. The13C KIE lying in the range from 1.0541 (at 18.4°C) to 1.0463(at 100.1°C) indicates that the rupture of the carbon—oxygen single bond is the reaction step which determines the13C fractionation in the observed production of carbon monoxide from formic acid assisted with Ac2O similarly to what has been observed in the decarbonylation of pure liquid formic acid. The magnitude of the13C KIE (smaller than the value expected for “full C—O bond rupture”) indicates again that there are intrinsic side processes lowering the13C fractionation in the course of decarbonylations of formic acid. The mechanism of decarbonylation of FA in the liquid medium consisting initially of “HCOOH–Ac2O” has been discussed and the most probable main and side minor reaction channels have been selected. The mechanistic conclusions have been corroborated by presenting the supplementary13C KIE data for CO formation in a “HCOOH–Ac2O–EDS” system, where the “overall13C KIE” is observed (EDS = etidronate disodium). The presented isotope data and their interpretation contribute to the full understanding of the decarbonylation of formic acid.