13C kinetic isotope effect (KIE) in the decarboxylation of phenylpropiolic acid (PPA) in tetralin medium (Tn) has been determined at 409-432 K and found to be of magnitude similar to the 13C KIE observed in the decarboxylation of malonic acid where the rupture of the C-C bond is the rate determining step. 13C KIE equals 1.0318/at 136 degrees C in the decarboxylation of PPA in Tn medium. Intramolecular 13C KIE in the decarboxylation of malonic acid equals 1.0316 at this temperature. Thus it has been shown that the nearly "full" 13C KIE can be achieved by providing the excess hydrogen to Calpha of PPA (or to triple acetylene bond) using not only strong mineral acids as the source of protons but also by carrying out the decarboxylation in organic medium like tetralin. A mechanism of decarboxylation of PPA in Tn is suggested.