The structure of caustic aluminate solutions in relation to the precipitation of gibbsite was investigated using vibrational spectroscopy and molecular dynamics simulations. Results from the molecular dynamics simulations indicate that aluminate ions form clusters as a function of time and that these clusters are stabilised by sodium ions. While the method used has the limitation that bond formation is forbidden, the predicted clustering would certainly facilitate polyaluminate anion formation. It is proposed that observed additional bands in vibrational spectra of concentrated aluminate solutions, as compared with those of dilute solutions, result from vibrations of these clusters (and any polyaluminate ions which arise from them). The absence of spectral features characteristic of a distinct interfacial aluminate layer at the growing crystal surface is explained by clustering throughout the bulk solution, and the participation of such clusters (and polyanions) in the growth process.