The grain structure of a commercial multifilamentary Nb3Sn wire has been investigated as a function of reaction temperature, TR, and time, tR. Results allow a quantitative description of grain growth. Based on the dependence of critical current, Nb3Sn area and grain size on the heat treatment conditions a two-stage reaction process is proposed to improve the critical current carrying capacity in magnetic fields greater than 10 Tesla. The first reaction converts the major part of the Nb to fine grained Nb3Sn at low TR. Subsequent high temperature ( 800°C) heat treatment for a short time ( h) increases both Tcand Bc2due to composition approaching stoichiometry (or a higher degree of order) without detrimental grain growth. First experimental results show a remarkable improvement in the high field critical current for this two stage-reaction process.