The hydrothermal synthesis of the aluminum phosphate Na2Al7(PO4)(12).4trenH(3).Na(H2O)(16) (MIL-74) has been studied by X-ray diffraction (conventional source) and in-situ time-resolved energy-dispersive X-ray diffraction (synchrotron radiation). Before the crystallization of MIL-74, the reaction of the starting aqueous aluminophosphate gel containing the tren species (tris(2-aminoethyl)amine) and sodium hydroxide gives rise to the formation of a hydrated chainlike solid Al(PO4)(2).trenH(3).6H(2)O (phase 1), which transforms into the hydrate Al(PO4)(2).trenH(3).2.5H(2)O (phase 2) upon heating at 180 degreesC. Its structure was determined by means of single-crystal X-ray diffraction and consists of a one-dimensional inorganic chain of tetrahedral unit AlO4 alternating with the phosphate group PO4. The resulting ([Al(PO4)(2)](3-))(n) chains interact through hydrogen bonds with the trenH(3) and water molecules, generating a layerlike inorganic-organic network. Phase 2 was observed to be an intermediate, which disappears when the MIL-74 solid starts to crystallize. The observation of such an intermediate phase allows for the proposition of a chemical reaction pathway for the formation of the three-dimensional network of MIL-74. It seems that the sodium cations play a crucial role and directly react with the chainlike precursor. They would participate in the construction of the inorganic framework as anchorage nodes during the condensation process.