Quantitative structure activity analysis of the substrate types Ala-Ala-AR and Ala-Pro-AR containing different substituents in the aryl ring showed that the rate-limiting step in the hydrolysis of the alanine substrates by dipeptidyl peptidase IV occurs in th acylation reaction (kcat approximately k2). Probably, the tetrahedral intermediate of the acylation process has a real life time. The positive q-value of the Hammett-equation in k'cat suggests that the N-atom of the arylamide is charged more negatively in the transition state TI not equal to than in the original state TI. The analysis of the quantitative conformation activity relationship (QCAR) gives information on the steric situation in the tetrahedral intermediate of the acylation step near the transition state. The rate limiting step in the hydrolysis of the substrates of the proline type occurs in the deacylation reaction.