A boundary layer fractionation model for the thermal and compositional evolution of a basaltic magma chamber is presented. The model utilizes a multicomponent thermodynamic approach for relating the compositional structure with the thermal structure of a cooling magma body. The magma composition in the main magma body evolves by transportation of a fractionated interstitial melt from the mushy boundary layer. The consideration of both the thermal and compositional evolution of the magma body enables a realistic simulation of magmatic differentiation as a function of time and space. The model is used to evaluate a primary magma estimation from volcanic rock series. It is shown that the common procedure of olivine addition gives an estimation which is significantly different from the true primary composition if the magma actually differentiated by boundary layer fractionation. This can cause significant errors in estimating the conditions at which the primary magma was generated. ¿ 1999 American Geophysical Union