The Coppi‐Perkins theory of collisional drift waves in low‐&bgr; plasmas is extended to plasmas containing ions of two different types. Recent experiments call for such an extension. Before discussing the general results, the specialization to the usual plasma shows the destabilizing effect of a high ionic charge and of an electron temperature exceeding the ion temperature. As an application of the obtained criterion of marginal stability the average ion mass is computed and compared to the intuitive linear formula, based only upon concentrations. Under present‐day conditions almost no difference is detected, but for future experiments with higher temperatures and higher mass ratios the correct formula is shown to give less stable results than the intuitive one at the same relative concentration. This is also true for lower densities and shorter wavelengths. The discrepancies can become quite important.