Utilizing a phase equilibria based trace element isotope model for low pressure igneous differentiation, it is now possible to update the methods presently being used to evaluate the effect of assimilation and fractional crystallization (AFC) processes. The results of the calculations presented here indicate that bulk partition coefficients (D) for Sr and Nd are strongly dependent on the chemistry of the assimilant and the rate of mixing. Virtually all silicic assimilants will increase bulk D for network-modifying cations because of the positive correlation of most trace element partition coefficients with reciprocal temperature, and the amount of network forming components in the melt. In addition to the effect of temperature and melt composition, the addition of a magma of different composition from the magma chamber will generally cause a change in the fractionating mineral proportions. For example, a peraluminous assimilant will increase the proportion of plagioclase, and a peralkaline assimilant will increase the proportion of augite. This will in turn have an effect on bulk D. For most multiply saturated mafic to intermediate systems differentiating at low pressure, bulk DSr is >1, and bulk DNd is <1. Thus the effect of a peraluminous assimilant is to drive bulk DSr away from 1 and a peralkaline assimilant drives bulk DNd toward 1. Therefore a peralkaline assimilant will generally drive the liquid path away from a simple mixing curve (or D=1 AFC curve) and a peralkaline assimilant will drive the liquid path toward a simple mixing curve. Compared to the results derived in this study, existing models using fixed bulk Ds calculated from closed system conditions will generate results that underestimate the assimilation rate, 87/86Sr and bulk Sr content for a peraluminous assimilant, and over-estimate the assimilation rate, 143/144Nd and Nd content of the assimilant for a peralkaline assimilant. ¿ American Geophysical Union 1989