Effects of the substrate and the coenzyme on the crystalline yeast phosphoglyceric acid mutase activity have been investigated. Lineweaver-Burk plots at different concentrations of the substrate (D-3-phosphoglyceric acid: 3×10-4 to 8×10-3M) and the coenzyme (D-2, 3-diphosphoglyceric acid: 8×10-7 to 10-5M) change in such a way to indicate the involvement of an enzyme-substrate-coenzyme ternary complex as an active intermediate in the enzymic reaction process. It is concluded that the reaction catalyzed by the yeast enzyme follows the sequential pathway and that a phosphorylated enzyme does not participate as an obligatory intermediate in the reaction mechanism, if it occurs. Kinetic studies indicate Km values of 6×10-4M for D-3-phosphoglyceric acid and 8×10-7M for D-2, 3-diphosphoglyceric acid. The substrate is a competitive inhibitor of the coenzyme with a Ksi (inhibition constant) of 3.2×10-3M. The coenzyme inhibition is not observed at concentration tested. A kinetic treatment to determine the mechanism of the enzyme reaction from the experimental data which are obtaind in the range of inhibitory substrate concentrations is presented.