The mechanism of formation of two different cubic mesoporous silica materials formed with Pluronic triblock copolymers is investigated with in situ time-resolved small-angle synchrotron X-ray scattering, in situ time-resolved H-1 nuclear magnetic resonance, and time-resolved transmission electron microscopy. The materials studied are the micellar cubic (Im (3) over barm) SBA-16 formed with Pluronic F108 and the bicontinuous cubic (Ia (3) over bard) silica material formed with Pluronic P103 and NaI. The formation mechanisms of the two cubic structures are shown to be dissimilar. For the Im (3) over barm material, in the early stages of the synthesis, floes of unordered micelles are observed, but areas where the micelles have started to order are also present. With time, there is an increase in order; however, there is a coexistence of unordered micelles and ordered material all through this study. The bicontinuous cubic silica is formed via a different path. The system is phase-separated already before the addition of the silica source, which implies that a concentrated phase is present, acting as the structure director of the Ia (3) over bard structure. The results are compared with earlier reports on the formation of the hexagonal SBA-15 material.