A study of plateau slopes of self‐quenching Geiger‐Mu¨ller counters shows that no quenching vapor tested produced flat characteristics. On the assumption that the slopes were given by spurious counts arising from the gas discharge phenomena, a study was made of the controlling factors. Neither the quenching gas nor the vehicular gas positive ions were found to be significant. A study of the negative ions produced by molecular dissociation of the quenching gas at the cathode, and electron attachment in the body of the gas shows that these factors are directly correlated with the slopes of the plateau characteristics.