The microstructural development and phase-transformation kinetics of stoichiometric cordierite glasses containing B2O3 and/or P2O5 additives were highly affected by the microstructural characteristics of the mu-cordierite and the type of additives. The addition of B2O3 tended to cause the formation of mu-spherulitic dendrites with thin dendritic arms, which promoted the formation of alpha-cordierite, either from crystallization of the residual glass or from transformation of mu-cordierite. P2O5 had the opposite effect: Increasing the temperature increased the growth rate of alpha-cordierite more than that of mu-cordierite.