We have used molecular simulations to examine the diffusion of Ne, Ar, Kr, Xe, CH4, CF4, CCl4, SF6, SnCl4, and SnBr4 in the molecular sieve AlPO4-5, both as single species and as coadsorbed mixtures. Single adsorbed species exhibit a transition from normal to single-file diffusion as a function of increasing adsorbate size. In addition to normal and single-file diffusion, coadsorbed mixtures exhibit a qualitatively new diffusion mode in which one species performs normal diffusion while the other undergoes single-file diffusion. We discuss the prospects for experimental verification of this phenomenon, and present measurements of the loading dependence of diffusion rates during the dual-mode diffusion of Ne/CF4 mixtures. (C) 1997 American Institute of Physics.