The annihilation rate of positrons in argon gas has been measured over the temperature range of 135 to 573 K with applied d.c. electric fields per unit gas density up to 35 V cm−1 amagat−1. An analysis of these data was carried out assuming that the annihilation rate and the momentum-transfer rate can both be approximated by functions of the form AνB over this velocity range. Results of the analysis indicate a strong velocity dependence for both the annihilation rate and the momentum-transfer rate. For velocities in atomic units [Formula: see text], the annihilation rate could be represented by va(ν)/D = (1.195 × 106) ν−0.480 s−1 amagat−1 over the velocity range: 0.03 to [Formula: see text]. The momentum-transfer rate was similarly determined as ν(ν)/D = (2.11 × 1011) ν−0.49 s−1 amagat−1. The statistical errors in these results are velocity dependent, with the largest being 16%. A modified effective range parameterization of the momentum-transfer cross section gives a zeroth-order scattering length of A0 = (−4.4 ± 0.5)a0 for positron–argon collisions.No evidence of a velocity dependence in the orthopositronium quenching rate at room temperature was detected.