We investigate the dynamics of untrapped molecules within a far-off resonant accelerating optical lattice. Our analysis shows that untrapped molecules can be temporarily transported by the lattice, and those that are transported for the longest time reach a unique, well-defined critical velocity that depends on the mass to polarizability ratio of the molecular species. We show that this species-dependent critical velocity leads to a velocity dispersion for different species within a gas mixture. Our numerical simulations show that the velocity distribution of a multicomponent gas evolves to form well-separated peaks in velocity space for each species. We propose a time-of-flight analysis technique that transforms the velocity dispersion to a temporal separation of different species, even for small differences in the mass to polarizability ratio. Separation utilizing this concept is demonstrated for atmospheric species and isotopes of nitrogen. Finally, we present an extension of this concept for both temporal and angular dispersion. (C) 2003 American Institute of Physics.