The carbon-13 isotope effect accompanying decarboxylation of phenylpropiolic acid in 85% orthophosphoric acid has been studied in the temperature range 323-353 K. The first fraction of CO₂ was found to be generated at 323 K in 85% H₃PO₄ with the carbon-13 isotope effect, k₁₂/k₁₃, equal to 1.011-1.015. The subsequent fractions of carbon dioxide produced in the temperature range 323-353 K have shown none but about 0.2% per cent kinetic isotope effect (KIE). It has been suggested that the initial larger carbon-13 fractionation, accompanying the decarboxylation of PPA in 85% orthophosphoric acid, is caused by isotopic equilibra linking decarboxylating monomeric species of PPA with dimeric and larger units formed during solvation and dissolving of solid PPA in orthophosphoric acid. The solvated PPA molecules are decarboxylated in the homogeneous solution with a negligible carbon-13 KIE. The triple bond of phenylpropiolic acid is attacked by the hydrated protons, H₃OĹaq, and formation of the C_alpha-H bond is the rate determining step. The bond rupture between the alpha carbon and the carboxyl group is the next fast reaction step. The enthalpy of activation, DH^ą = 123.4 kJ/mol, and the positive entropy of activation, DS^ą = +30 e.u.(SI), exclude the chelate like structure of the transition state. This suggests a flexible structure of the transition state complex which must be less solvated than in the initial state. Some further studies of the carbon-13 KIEs in phosphoric acid environment are suggested.