This paper focuses on the influence of the addition of ammonium molybdate to geopolymer mixtures to better understand the formation of geopolymer structural networks and the effect of Mo-based additive on the thermal behavior of material. Several geopolymers were synthesized between two metakaolins and various amounts of ammonium molybdate. The microstructure of samples was determined by SEM, the structural evolution (depending on the amount of molybdenum added) was investigated using Raman spectroscopy and XRD analyses, and the temperature behaviors of materials by DTA-TGA and dilatometric analyses. The results showed that (i) both metakaolins react differently in the presence of ammonium molybdate, (ii) molybdenum induces decrease of shrinkage value at high temperatures, and (iii) the crystallization seems to be favored with ammonium molybdate. This work shows that ammonium molybdate controls polycondensation reactions highlighting the modifier or former role played by the molybdenum atoms depending on the amount of ammonium molybdate added. (C) 2015 Elsevier B.V. All rights reserved.