This paper presents a model of water flux and throughfall concentrations of K+ and NH4+ in a subalpine balsam fir forest. The model is based on a multi-layer submodel of hydrologic flow. Cloud water deposition and evaporation are incorporated as separate submodels. Chemical exchange is parameterized with diffusion resistances and internal foliar concentrations determined from leaching experiments on isolated canopy components. The model is tested against within-storm throughfall measurements and found to agree reasonably well in most instances. Some specific departures from observed data are noted, of which some can be explained. Differences between observed and modeled concentrations of K+ early in the storm events suggest that pre-storm conditions, which were not modeled, are important in controlling the chemical exchange. Responses of throughfall chemistry to changes in rain rate, rain concentration, and stand surface area index (SAI) were investigated by simulation with the model. Increasing rain rates increased leaching of K+ and uptake of NH4+. Increasing concentrations of K+ in rain decreased slightly the amount of K+ leached, but increasing concentration of NH4+ in rain increased NH4+ uptake proportionately. Increasing canopy SAI increased the leaching of K+ and the uptake of NH4+, with the pattern of the increase dependent on rain rate.