THE Ziegler–Natta polymerization of α-olefines has been widely investigated by many workers mainly from the practical point of view. On the other hand, the details of the reaction mechanism are not yet clear. There are, of course, some basic investigations of the reaction by Natta et al.1, one of which is considered here; that is, the kinetic behaviour of the polymerization of propylene in the initial stages (Natta's ‘the adjustment period’), where the polymerization-rate, r, changes with the polymerization time, t, from the initial rate, r0, at t = 0, to the stationary rate, r∞. The phenomenon can be divided into two types, that is, the build-up type and the decay type. The decay type was observed with the ground catalysts (sizes ⩽ 2µ), while the build-up type was observed with the unground ones (1∼100µ). Analysing the latter types of rate change, Natta et al. found that: where tN is the polymerization time at which the rate equals ¾ of the value of the stationary rate, r∞, and P the pressure of propylene. Natta suggested that the stationary reaction corresponds to a definite particle size of the catalyst, and any other catalyst with a different particle size would alter to this definite size.