In geopolymeric gels, aluminum is known to have tetrahedral coordination and to be associated with an alkali cation to satisfy electronic neutrality. However, the mechanism of aluminum incorporation and the effects of different synthesis parameters are not well understood. H-2, Al-27, and Na-23 MAS NMR techniques have been used to elucidate the structural properties and mechanism of aluminum incorporation in metakaolin-based geopolymers. Na+(aq) and Al(OH)(4)(-)(aq) species were detected in the pore solution of geopolymers with Si/Al ratios of less than or equal to 1.40 and found to be directly related to the concentration of soluble silicate in the activating solution used for synthesis. The relative concentration of Na+(aq) in sodium and mixed-alkali geopolymers indicated a preferential incorporation of potassium into geopolymeric gels during curing. This work demonstrates a direct link between the composition of the silicate activator and aluminum incorporation in geopolymeric gels.