The Michaelis constants of yeast pyruvate decarboxylase at full substrate activation and the maximum decarboxylation rates were determined from a total of 21 aliphatic and aromatic α-oxo acids in the absence of buffer substances. By means of a pyruvamide-activated enzyme it was possible to confirm experimentally the Km-values obtained by extrapolation. The substrate binding rate of pyruvate decarboxylase increases linearly with increasing H+ concentration, which indicates that only the undissociated pyruvic acid acts as the substrate of the enzyme. Whilst the substrate binding rate can be correlated linearly with the steric substituent constants of aliphatic α-oxo acids, it was not possible to find a corresponding relation between the aromatic α-oxo acids and the Hammett σ-constants, which indicates steric peculiarities in the active centre of pyruvate decarboxylase. On the other hand, a linear relation between the maximum velocities and the σ+-values for direct conjugation was established, leading to some conclusions concerning the mechanism of the splitting-off of aldehyde.