Acid-based geopolymers exhibit interesting properties, such as higher temperature resistance (up to 1450 degrees C) and better mechanical properties, as compared to alkali-based geopolymers. The aim of this work was to highlight the origin of water and thermal resistances of acid-based geopolymers, demonstrated in our previous study, by Al-27, Si-29 and P-31 NMR and XRD experiments. Different networks were indeed identified in consolidated materials, such as Al-O-P, Si-O-P, vitreous silica and Si-O-Al (only for Al/P = 4, due to unreacted metakaolin) depending on processing conditions. The final properties are induced by the proportion of these networks which are controlled by the Al/P ratio and the setting temperature. It was shown that a higher amount of Al-O-P bonds and the absence of unreacted metakaolin conduct to a thermally resistant sample. A more organized structure with a low proportion of Q(0)(1Al) and a preference for Q(1)(2Al), as seen by P-31 NMR, induces a great water resistance of geopolymers. After heating at 1000 degrees C, different AlPO4 polymorphs crystallize, as well as mullite in the case of Al/P = 4.