Measurements have been made of pre-breakdown currents in four gases: oxygen, air, freon-12 (C${\mathrm{Cl}}_2$${\mathrm{F}}_2$), and C${\mathrm{F}}_3$S${\mathrm{F}}_5$. The behavior of these currents differs from that previously attributed to all gases in that semilogarithmic graphs of current against electrode separation at constant $\frac{E}{p}$ exhibit initial curvature, being concave toward the electrode separation axis. At low $\frac{E}{p}$ an apparent saturation is found. This behavior is interpreted as resulting from a reaction of dissociative attachment of the kind $AB+e={\left(A{B}^{-}\right)}^{*}=A+B^{-}$. On this basis simultaneously determined values of the ionization and attachment coefficients can be deduced. In oxygen and air these parameters, when determined from separate experiments, show a somewhat anomalous behavior. The apparent anomalies are explained with the result that ionization is found to commence at lower, and attachment persist to higher values of $\frac{E}{p}$ than previously suspected. In freon-12 and C${\mathrm{F}}_3$S${\mathrm{F}}_5$ it is found that the ionization coefficient is about an order of magnitude larger than reported for similar gases when no account is taken of possible attachment. The attachment coefficient is observed to be larger for halogen-containing molecules than for the same halogens when free. Cross sections for attachment are computed for oxygen and air. Dependence of the cross sections on mean electron energy is discussed with reference to energy scales in these two gases.

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