This paper updates three previous papers entitled ''Transport Properties of Gaseous Ions over a Wide Energy Range.'' These papers, referred to as Parts I, II, and III, were by H. W. Ellis, R. Y. Pal, E. W. McDaniel, E. A. Mason, and L. A. Viehland, ATOMIC DATA AND NUCLEAR DATA TABLES 17, 177-210 (1976); H. W. Ellis, E. W. McDaniel, D. L. Albritton, L. A. Viehland, S. L. Lin, and E. A. Mason, ATOMIC DATA AND NUCLEAR DATA TABLES 22, 179-217 (1978); and H. W. Ellis, M. G. Thackston, E. W. McDaniel, and E. A. Mason, ATOMIC DATA AND NUCLEAR DATA TABLES 31, 113- 151 (1984). Part IV contains compilations of experimental data on ionic mobilities and diffusion coefficients (both longitudinal and transverse) for ions in neutral gases in an externally applied electrostatic field, at various gas temperatures; the data are tabulated as a function of the ionic energy parameter E/N, where E is the electric field strength and N is the number density of the neutral gas. Part IV also contains a locator key to ionic mobilities and diffusion coefficients compiled in Parts I-IV. The coverage of the literature extends into 1994. The criteria for selection of the data are: (1) the measurements must cover a reasonably wide range of EIN; (2) the identity of the ions must be well established; and (3) the accuracy of the data must be good. (C) 1995 Academic Press, Inc.