Experiments began with ''neutralization'' of iron (III)-aluminum nitrate solutions by addition of sodium hydroxide at room temperature. The final pH of most experiments was approximately 13.7. The samples were neutralized in PP or HDPE bottles that were then submerged in a water bath and aged at 90 C for up to 120 h. (In the low-Al systems, X-ray diffraction (XRD) results suggest that transformation of the initial amorphous precipitate to crystalline phases was complete after this time.) Experiments were run either as a single 250 mL sample (''single age experiments'') or as 2 L batches, from which 250 mL aliquots were drawn (''batch experiments''). Both solid phase chemistry and solution composition were monitored throughout the aging process. Samples were centrifuged and the experimental supernatant solutions were decanted and passed through a 0.4 mm syringe filter prior to analysis. Solids were dialyzed to removed excess salts. Aqueous Al and Fe were measured by the catechol violet (Dougan and Wilson, 1974) and Ferrozine (Dawson and Lyle, 1990) techniques, respectively. X-ray diffraction data were taken with a Scintag powder diffractometer scanning at 1{sup o} 2Q per minute using Cu K-a radiation and a graphite beam monochromator.