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Total cross sections (Qr's) have been measured for I -500-eV positrons and electrons scattered by 02, CH4, and SF6 using a beam-transmission technique. The positron Qr s are compared with the corresponding electron Qr's for each target gas. It is found that the positron Qr's are, in general, lower than the electron results. There are no prominent structures observed for positron scattering at low energies that would be comparable to the narrow shape resonances observed for electrons scattering from various molecules, such as SF6. The positron Qr curve for CH4 reveals a significant increase in the vicinity of its positronium formation threshold, while the 02 and SF6 curves are monotonically increasing in this vicinity. At the highest energies investigated, there are indications of a tendency toward merging of the positron and electron Qr s for these gases. Interesting similarities are found in a comparison of the present positron and electron Qr curves for CH4 with prior comparison measurements for argon.

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