The paper deals with a method of the control of counting gas purity and continuous examination of the counting efficiency during measurements of natural radiocarbon in carbon dioxide filled proportional counter. The described method is based on the registration of the number N of coincidence muon counts between the two sections of GM guard counters in addition to the commonly used recording of the number L of coincidence counts between the proportional counter and the guard counters taken as a whole. It has been shown that the ratio C= L/ N depends only on the proportional counter performance and resembles its overall counting efficiency. The method was checked on three samples of inadequate purity measured in the Gliwice Radiocarbon Laboratory and the results have been found satisfactory.