AbstractFor a series of homogeneous ethylene‐propylene copolymers with mole fraction ethylene (x1) ranging between 0.40 and 0.85, the inversion (I) independent propagation probabilities (Pijs) and reactivity ratios ( rijs) and the methylene, ethylene, and propylene sequence length distributions have been determined from 13C nuclear magnetic resonance (NMR) data, using a first‐order Markovian terpolymer [ethylene (1), inverted propylene (2), and normal propylene (3)] model. For each sample, the limiting values of I (Imin, Imax) are given. Calculations of the common parameters for 19 samples show that the polymerization direction characterized by the r set and I is statistically more probable than the opposite direction, which is characterized by the r′ set and I′. Further, I = Imin appears to be close to the most probable value of I. The resulting r set is r12 = 119, r13 = 19.7 and, for I = Imin, r21 = 0, r31 = 0.034, r32 = 2.98. In the limited range 0.60 < x1 < 0.85, there appears to be no preference for either polymerization direction, so that the solutions characterized by the r set and I = Imin and by the r′‐set and I = I′max, respectively, are about equally probable. If the copolymer reactivity ratios re and rp are defined in the usual manner, it follows that for the limited range re = 17. In the model used here, rp is found to be dependent on the ethylene‐propylene ratio in the reactor. Nevertheless, it can be stated that rp ≅ 0.029 and re rp ≅ 0.50.