The effect whichSiF4,SO2, andSF6have on the plateau and the pulse size distribution of a proportional counterwere investigated.SiF4was tested at three pressures, 30, 45, and 60 cm, ofBF3and its effect was found to be independentof the counter pressure for the range of value considered. From the variation in the plateau, the attachmentprobability forSiF4was calculated to beh = 1.485×10−5and its cross section for attachment to be&sgr;a = 5.12×10−20 cm2. The amount of these gases permitted, without the counter being affected beyond the limitsof tolerance which are set up, were found to be 0.04% forSiF4, 0.01% forSO2, and2.0×10−6%forSF6.