In this first-paper of a series of publications on the mechanism of the polymerization of ¿-caprolactam (CL), an investigation of the kinetics of the polymerization initiated by ¿-aminocaproic acid (ACA) is reported. Experiments were performed at 254°C. for three different initial ratios (ACA/CL)0:0.05, 0.022, and 0.011 mole/mole. The concentration (x) of unconverted CL and that of end groups (c) were determined by extraction with water at room temperature and by potentiometric titration, respectively. The method of kinetic evaluation consists in the direct checking of rate equations making use of the quantities dx/dt and dc/dt which were determined graphically from the curves of x and c versus time. In this way tedious integration procedures are avoided and the testing of rather different mechanisms is much easier. Moreover, it is not necessary to make the assumption of constant rate coefficients which, of course, will seldom apply. Thus the inadequacy of the "polycondensation mechanism" has been proved for every time instant separately, although accidentally the rate constant of the main reaction is fairly constant. In this polycondensation mechanism it is assumed that polymer formation can take place only by polycondensation of ACA, the supply of which is provided for by the reaction of CL with water formed in the process of polycondensation. It turns out that even when the rate of hydrolysis of CL is so high that the polycondensation reaction is rate determining, this latter rate is to small by a factor of 10 or more to account for the rate of CL conversion observed. It is shown that this difficulty can be overcome only by incorporating a third reaction in the mechanism, a reaction which must provide for the main consumption of CL. We have assumed this reaction to be a reversible polyaddition reaction, i.e., a reaction in which CL is added to one of the end groups of the polymer molecules. The rate constant for this reaction derived from the experiments appears to be in good agreement with the value derived from the experiments on the polymerization of CL initiated with water.