In Section I a general formula for $\ensuremath{\alpha}$, the coefficient of recombination of ions in gases, is developed. It covers all ranges of pressures and temperatures and can be made to include all types of recombination (preferential, initial and volume ionization). The evaluation of the general formula, (1.1), depends on the relative values of three linear quantities: the mean free path of the ions $\ensuremath{\lambda}$, the mean distance between ions of different signs $\overline{r}$ and the well-known parameter ${a}_{0}=\frac{{e}^{2}}{(\mathrm{kKT})}$. In Section II the case $\ensuremath{\lambda}\ensuremath{\ll}{a}_{0}$ is treated. The mechanism of recombination then depends on the ratio $\frac{{a}_{0}}{\overline{r}}$. If $\frac{{a}_{0}}{\overline{r}}$ is large the migration of the ions under their mutual attraction prevails and (1.1) leads to Langevin's formula; in the opposite case, $\frac{{a}_{0}}{\overline{r}}\ensuremath{\ll}1$, diffusion is the decisive feature and 1.1 leads to a formula which is practically identical with that of Harper. In Section III the case $\ensuremath{\lambda}\ensuremath{\gg}{a}_{0}$ is treated by a method previously developed by the author. Under certain restrictions (1.1) reduces to Thomson's formula, but in general $\ensuremath{\alpha}$ depends on the concentration of the ions. In Section IV it is shown that (1.1) is in fair agreement with such experimental data as are available for the region of transition between the cases treated in Sections II and III.