Solid rocket propellant combustion is hindered by agglomeration of aluminum particles on its burning surface and determining the particle size has been a problem for half a century. The actual size of the agglomerates at motor pressures is unknown due to the opacity of the combustion plume, particularly at the elevated pressures seen in operational rocket motors. Sampling techniques can provide data at elevated pressure but may be biased due to the sampling method and do not provide information on the dynamics of agglomerate formation. This study uses time-resolved synchrotron x-ray imaging (with both absorption and phase contrast) to view aluminum agglomerate formation in situ at relevant rocket pressures. We have for the first time observed agglomerate formation at motor-relevant pressures in real time with unprecedented fidelity, providing critical data for understanding the combustion of aluminized solid rocket propellants. Published by Elsevier Inc. on behalf of The Combustion Institute. Published by Elsevier Inc.