The structures of gaseous CH3AsF2 and (CH3)(2)AsF have been determined by electron diffraction incorporating vibrational amplitudes derived from ab initio force fields scaled by experimental frequencies and, for the difluoride, restrained by microwave constants. The following parameters (r(alpha)(o) structure, distances in pm, angles in degrees) have been determined for CH3AsF2 : r(As-C) = 194.6(4), r(As-F) = 173.1(1), angle CAsF = 95.2(1), angle FAsF = 97.0(1). For (CH3)(2)AsF structural refinement gives r(As-C) = 195.1(1), r(As-F) = 175.4(1), angle CAsF = 95.3(5), and angle CAsC = 96.9(8). For the series (CH3)(3)As, (CH3)(2)AsF, CH3AsF2, and AsF3, both As-C and As-F bond lengths are shortened with increasing numbers off atoms, but the angles CAsF and FAsF are almost invariant.