The reduced Townsend ionization coefficient α/P is derived from the measurement of the mean gas amplification factor in a low-pressure (P ≤ 50 kPa) proportional counter filled with argon–propane and argon–(dimethyl-ether) mixtures of various concentrations. The results are compared with our previous α/P values for argon–isobutane mixtures [I. Krajcar Bronić and B. Grosswendt, Nucl. Instrum. Methods Phys. Res. B 142, 219 (1998)]. The range of the reduced electric field strengths was 7×104 V m-1 kPa-1≤Sa≤4×106 V m-1 kPa-1. In such strong electric fields electrons do not reach the equilibrium with the electric field and the reduced gas gain depends on the gas pressure. For the analysis of the experimental gas gain data we used the field gradient model of Townsend–Williams–Sara–Segur. We determined the pressure dependent coefficients A* and B* of the Townsend ionization coefficient α/P=A*×exp(-B*/Sa). We found that the coefficients increase (i) if the admixture concentration increases at constant pressure, and (ii) if the pressure decreases at constant admixture concentration. In both cases the coefficients increase with the increase of Sa. The effective ionization potential of the mixtures, Vi*=B*/A*, at low admixture concentrations reaches the ionization potential I of the admixture as a consequence of the transfer of energy from the excited metastable states of argon to ionization of the admixtu- re through the Penning effect. When α/P vs Sa curves for mixtures of different concentrations at constant total pressure are compared, crossings of the α/P curves at a certain range of Sa values are observed in all kinds of mixtures and for all pressures. We show the necessity of the crossings by taking into account the mathematical properties of the function describing the ionization coefficient. © 2000 American Institute of Physics.