The motion of ions in moderately dense gases under the action of an electrostatic field is simulated through a nonequlibrium molecular dynamics method. The method is developed through consideration of multiple ion-neutral collisions in a previously established procedure for low-density gases. The first two moments of the ion velocity distribution function for the representative system of K+ in Ar are calculated at various gas densities and field strengths and through them the mobility and two effective temperatures, parallel and perpendicular to the field. Additional tests for the accuracy of analytic expressions for the effective temperatures in terms of drift velocity and differential mobility derived from a three-temperature treatment of the Boltzmann kinetic equation were successful supporting the extension of use of generalized Einstein relations in this area. The procedure is easily extendable to the case of molecular ions with internal degrees of freedom. (C) 2003 American Institute of Physics.