The paper describes experiments made with Geiger-Müller counters in which the cylinder potential is positive with respect to the wire instead of negative as ordinarily used. The counter operates under reduced efficiency with this reversed condition. When used as a double coincidence set the reduction in efficiency is greater than for a single counter. The paper also describes experiments confirming a view that the inefficiency of the reversed counter may be explained by supposing the counter action to be produced when a positive ion strikes the wire and liberates from it a secondary electron. Whenever such an electron is liberated a discharge takes place, but the probability of liberation is low. However, it seems reasonable that the probabilities of such liberation of secondary electrons as found by Penning and others stands in agreement with the efficiency observed in the counter experiments cited above.