Basaltic magma is the most widely used probe of mantle chemistry and structure. Chemical characterization of the sources for mantle-derived magma is possible only after quantitative evaluation of possible contamination by crustal wall rocks. The most commonly invoked contamination mechanisms are the combined processes of crustal assimilation and magmatic fractional crystallization (AFC). Because of the energetic link between these processes, predicting assimilation and crystallization progress requires modeling both heat and mass transfer in the magma–country-rock system. Using the MELTS software package, we modeled isenthalpic (heat-balanced) AFC between two common types of basaltic magma and several types of felsic crust at 1 kbar. Our simulations show that during the early stages of isenthalpic AFC, the ratio of rates of assimilation to crystallization (r) may be substantially greater than one (2.0–2.7), allowing assimilation of a mass of country rock up to 5%–18% of the initial magma mass with only 3%–7% crystallization. The second stage of AFC, beginning with plagioclase and/or pyroxene saturation, is characterized by lower r (0.5–1.0). The initial high-r stage results from suppression of crystallization associated with the change in magma composition as assimilation progresses. Under certain conditions, even small degrees of crystallization of olivine alone, coupled with cooling of the magma, can accommodate relatively large amounts of crustal assimilation, and cause large shifts in isotopic and trace element geochemical indices with little differentiation.