The deposition of films within a porous substrate is modeled in a countercurrent reactor configuration. Two transient models are solved: a diffusion-one-step reaction chemistry model and a diffusion-four-step reaction model that incorporates precursor formation followed by nucleation and growth events. It is shown that the quasi-steady-state/infinitely fast reaction assumptions employed in diffusion-reaction models are not valid in either the slow or fast kinetics regime due to the transient evolution of a deposit. The diffusion-one-step reaction model results in continuous deposits, which are relatively thick. The interplay of nucleation and growth with diffusion and reaction can result in quite thinner deposits. The demonstrated strong dependence of deposition features on initial conditions provides an additional means of controlling the deposition on one hand but indicates possible reduced experimental reproducibility on the other. Application to Pd membranes in alumina under supercritical CO2 conditions is discussed.