A linearized model of an ideal diatomic gas with diffusion processes and rate reaction is applied to one-dimensional unsteady motion generated by initial nonuniformities of state and velocity. The time-dependent system resembles that of steady free-mixing of jet or wake type, with two space coordinates. The temporal and spatial character of the profiles of flow variables is studied analytically under two sets of conditions. First considered is the case of unit Lewis number and for this case the investigation is subdivided as follows: (i) Zero mass motion with state nonuniformities for planar, cylindrical, and spherical configurations; and (ii) nonzero mass motion with state nonuniformities for the planar and cylindrical configurations. Secondly, the case of arbitrary, but constant, Lewis number is considered. In this instance, the investigation is limited to nonzero mass motion with state nonuniformities in the planar geometry. In all cases the Prandtl number is assumed to be an arbitrary constant. Analytic solutions are obtained for all cases and various parametric effects discussed.