(1) Silicic acid alone does not bind A1 3 + (10 - 3 M) at pH values below 6.8, but above this an association is formed which is not broken by addition of the dye chrome azurol S. The precipitate which forms slowly between silicic acid and Al 3 + can be removed by centrifugation. At this concentration (10 - 3 M), Al 3 + alone precipitates close to pH 7.(2) Phosvitin binds Al 3 + at pH 5.0, with a K D of 10 - 8 M (E. Rowatt and R.J.P. Williams, J. Inorg. Biochem., 55 (1994) 249; T.P. Geladopoulos and T.G. Sotiroudis, J. Inorg. Biochem., 54 (1994) 247) with a ratio of Al 3 + to phosvitin of 70. The complex precipitates when about half the Al 3 + is added but redissolves at high concentrations of phosvitin.(3) Silicic acid does not remove Al 3 + from phosvitin and phosvitin does not remove Al 3 + from a mature precipitate of aluminosilicate, although it removes some Al 3 + if it is added to aluminium newly associated with silicic acid.(4) The relationships between aluminium, phosvitin and silicic acid can only be explained by taking into account the kinetics of aging of aluminium-silicic acid precipitates.