¹³C isotope effects in the decarbonylation of formic acid of natural isotopic composition initiated by phoshorus pentoxide have been studied in a large temperature range (-5°C) - (+90°C). The ¹³C fractionation in the carbon monoxide production at -5°C increased from a low value of 1.2% characteristic of the first fractions of consecutively collected portions of carbon monoxide to higher values of ¹³C KIE observed in the decarbonylation of pure formic acid at corresponding temperatures. The temperature and time dependences of the measured ¹³C fractionations are functions of the relative number of millimoles of formic acid and the dehydrating phosphoric anhydride, P₂O₅. The addition of metaphosphoric acid reagent to unreacted formic acid containing H₃PO₄ significantly increased the ¹³C fractionation in subsequent decarbonylations at 70.4°C but to a slightly less degree than expected (¹³C KIE = 1.0503 instead of 1.0535). The addition of metaphosphoric acid reagent to formic acid saturated with NaCl results in the experimental ¹³C fractionation of a value of 1.0534 very close to the theoreticall one. An explanation of the low values of ¹³C KIE in the initial stages of HCOOH/P₂O₅ decarbonylations has been presented.