The synthesis of TBA- and DPA-VPI-5 has been studied in detail. Optimal synthesis conditions have been established using ternary composition diagrams for TBA-Al2O3-P2O5 and DPA-Al2O3-P2O. Gel samples at different aging times have been examined using chemical analysis, X-ray diffraction, FT-IR, and solid-state NMR. Two reactions occur between H3PO4 and pseudo-boehmite during gel aging. Interaction between the acid and the OH groups linking the pseudo-boehmite layers leads to the formation of an Al-O-P species in which Al is 6-coordinated. A further reaction between the acid and the Al-O-P species results in the formation of a 4-coordinated Al species. Because of the different aging conditions, the latter reaction begins much earlier in the TBA system than in the DPA system. The behavior of DPA in the synthesis gel is different from that of TBA. There is evidence that crystallization of VPI-5 proceeds via a solid-state mechanism, and that nucleation in both systems occurs already during the aging of the gels.