The present study examines the selective adsorption and polymerization of two amino acids, glutamic acid (Glu) and arginine (Arg), on a cationic clay mineral, montmorillonite (Mt). Two experimental procedures were used: selective adsorption and wet impregnation. In the first case, an adsorption selectivity is observed based on pH-dependent speciation of the amino acids. At natural pH, arginine is positively charged and thus extensively exchanges the cations in the interlayer space of the montmorillonite whereas glutamic acid is negatively charged and adsorbed in weak amounts, probably on the clay edges. In contrast, incipient wetness impregnation forces equivalent quantities of both amino acids to be deposited. After moderate thermal activation, combined characterization techniques, especially solid-state NMR and matrix-assisted laser desorption ionization time-of-flight analysis, highlight a peptidic condensation between the amino acids and hint at a selective polymerization yielding preferably heteropeptides (e.g., cyclo(Glu-Arg)) rather than homopeptides.