The formation of positronium in helium gas under an applied electric field has been investigated in order to derive a value for the momentum-transfer cross section, (σ m, of positrons on helium atoms. The major increase in positronium formation was noticed at Ɛ/D's of 60 V cmˉ¹ amagatsˉ¹ . Ɛ/D values up to 100 V cm-¹ amagatsˉ¹ were obtained. A minor increase in positronium formation was found at 30 V cm-¹ amagatsˉ¹ and was attributed to impurities in the helium gas, specifically N₂, 0₂ and/or H₂. Using the theory of Teutsch and Hughes a value of σ m of (0.25±0.03) π a ₀² was found by fitting to the upper increase. This corresponds to positron energies near 17.7 eV. Although the theory is sensitive to the positronium formation cross section, the combined effect of impurities and counting statistics left this parameter undefined. A fit was made to the lower positronium formation increase under the assumption that this increase is due entirely to the impurity atoms, whereas the positron velocity distribution is due to the helium atoms. This gave a value for the positronium threshold for the impurities of 8 ±1 e V. / Science, Faculty of / Physics and Astronomy, Department of / Graduate