Nuclear magnetic resonance (NMR) studies of geopolymer gels have shown directly that free Na cations are present in the pore solution of some specimens. Furthermore, it has been suggested previously but not directly proven that potassium is incorporated into these materials, in preference to sodium. Here, the presence of free potassium in the pore solution of geopolymeric gels prepared without sodium hydroxide was confirmed by K-39 NMR. The preferential incorporation of potassium was directly shown by the absence of free potassium in the mixed-alkali geopolymer gel.