A flowing afterglow apparatus has been used to measure Penning ionization cross sections for collisions of He 2 3S metastable atoms with Ar, Kr, Xe, H2, N2, O2, NO, CO, CH4, CO2 and NH3. Two different experimental methods have been employed. In the first the afterglow is modulated and the metastable density is measured by line absorption, while in the second this density is monitored by the ions produced in a second Penning reaction. Using the rigid-sphere approximation for the collision cross section, the ionization probability per collision is derived. In the cases of NH3 and CO2 this probability is near unity.