Experimental data on electron attachment and breakdown voltages V_s for two groups of strongly electron attaching gases-perfluorocarbons and freons-and some of their mixtures with electron slowing-down gases have been used in an effort to quantify the role of electron attachment on the breakdown strength of gases. The perfluorocarbons (PFC) considered have positive electron affinities (EAS) and capture electrons at thermal and near-thermal energies principally non-dissociatively. The freons capture slow electrons dissociatively producing mainly Cl^-. V_s is strongly influenced by the magnitude and energy dependence of the attachment cross-section, sigma _a( epsilon ). The increase in V_s with increasing attachment, however, seems to saturate, indicating an upper limit in V_s of approximately 2.5 times that of SF₆. Electron detachment seems to be indicated for PFC and SF₆.