Pu(IV) oxyhydroxide colloid growth is investigated with XAFS and LIBD. From combined results a model of colloid formation is proposed, which leads to a face-centered cubic Pu sublattice having cation defects, as observed with EXAFS, and a linear dependency of log [Pu(IV)] on -log [H+] with slope -2, in accord with LIBD. The solubility for Pu(IV) measured with LIBD is close to the lower limit of the solubility curve from previously reported data.