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Dynamic X-ray diffraction (DXRD) has been used in an effort to identify the specific phase changes which are responsible for observed thermal events at ∼980 °C in mullite gel precursors. Specifically, changes in the evolution of the common and strongest diffraction peak (d = 0.139 nm) corresponding to both transient alumina phases and the Al-Si spinel were followed in order to descriminate between these two phases. Results which compare the DXRD results for a diphasic mullite gel and a boehmite gel are presented and suggest that the Al-Si spinel phase forms at ∼980 °C in diphasic gels along with δ- and/or γ-Al2O3. These results are corroborated by separate TEM measurements which indicate the presence of both phases in samples quenched from 1000 °C.