In this experiment the charge produced in a large ionization chamber, by the complete stopping of ${\mathrm{Pu}}^{239}$ alpha particles, was measured using an accurately calibrated capacitor in series with a potentiometer. An electrometer connected across both the capacitor and potentiometer was used as a null indicator. From the charge produced the number of ion pairs $n$ was deduced and $W$, defined by the equation $W=\frac{E}{n}$ where $E$ is the energy used in forming the $n$ ion pairs, was computed.For pure gases, we found the following values of $W$ (in ev per ion pair): A, 26.4\ifmmode\pm\else\textpm\fi{}0.3; B${\mathrm{F}}_{3}$, 36.0\ifmmode\pm\else\textpm\fi{}0.4; ${\mathrm{C}}_{4}$${\mathrm{H}}_{10}$, 26.4\ifmmode\pm\else\textpm\fi{}0.3; C${\mathrm{O}}_{2}$, 34.3\ifmmode\pm\else\textpm\fi{}0.3; ${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$, 28.0\ifmmode\pm\else\textpm\fi{}0.3; He, 46.0\ifmmode\pm\else\textpm\fi{}0.5; ${\mathrm{H}}_{2}$, 37.0\ifmmode\pm\else\textpm\fi{}0.4; C${\mathrm{H}}_{4}$, 29.4\ifmmode\pm\else\textpm\fi{}0.3; ${\mathrm{N}}_{2}$, 36.3\ifmmode\pm\else\textpm\fi{}0.4; ${\mathrm{O}}_{2}$, 32.2\ifmmode\pm\else\textpm\fi{}0.3; S${\mathrm{F}}_{6}$, 35.7\ifmmode\pm\else\textpm\fi{}0.4; air, 35.0\ifmmode\pm\else\textpm\fi{}0.3; freon 12, 29.5\ifmmode\pm\else\textpm\fi{}0.3.Our results for $W$ for a mixture of gases are described by the formula $\frac{1}{{W}_{m}}=\left(\frac{1}{{W}_{1}}\ensuremath{-}\frac{1}{{W}_{2}}\right)z+\frac{1}{{W}_{2}},$ where $z=\frac{{P}_{1}}{({P}_{1}+a{P}_{2})}$, and ${W}_{m}$ is the $W$ for the gas mixture having two components of pressure ${P}_{1}$ and ${P}_{2}$ with $W$ values for the pure gases equal to ${W}_{1}$, and ${W}_{2}$, respectively. For the following mixture $a$ has the values shown: ${\mathrm{N}}_{2}$-${\mathrm{H}}_{2}$, 0.28; ${\mathrm{N}}_{2}$-A, 0.53; ${\mathrm{N}}_{2}$-${\mathrm{O}}_{2}$, 1.06; He-A, 0.75; He-${\mathrm{H}}_{2}$, 3.55; He-${\mathrm{N}}_{2}$, 8.47; He-C${\mathrm{H}}_{4}$, 0.68; A-${\mathrm{H}}_{2}$, 0.56; ${\mathrm{H}}_{2}$-C${\mathrm{H}}_{4}$, 4.03.In the case of He the $W$ of 46.0 is not used in the above equation, since small traces of impurities reduce $W(\mathrm{He})$ to 30.0 electron volts, and this latter value is appropriate for using in the formula for gas mixtures.