Vermiculite from Santa Olalla, Spain, was intercalated with tetramethylphosphonium [P(CH 3 ) 4 (super +) = TMP], using a TMP-bromide solution at 70 degrees C for three weeks. The resulting TMP-exchanged vermiculite, which contained a small (<5% of a site) amount of residual interlayer Ca, showed near perfect three-dimensional stacking. Cell parameters are a = 5.3492(8) Aa, b = 9.266(2) Aa, c = 14.505(6) Aa, beta = 97.08(2) degrees , space group is C2/m, and polytype is lM. Single-crystal X-ray refinement (R = 0.052, wR = 0.061) located two crystallographically unique sites for the phosphorus atoms (TMP molecule). The phosphorus atoms are occupied partially [P1 = 0.146(6), P2 = 0.098(5)] and are offset from the central plane of the interlayer by 1.23 Aa to form two P-rich planes in the interlayer. Electrostatic interactions between the P cations and basal oxygen atoms essentially balance the negative charge associated with A1 for Si substitutions in the tetrahedral sites. In addition, the orientations of the TMP molecules are probably different owing to packing constraints. The H 2 O site is located in the center of the interlayer, at the center of the silicate ring, and approximately 3.09 Aa from the Ca, which is also located on the central plane of the interlayer. Other H 2 O molecules are present in the interlayer, but could not be located by the diffraction experiment because they are randomly positioned in the interlayer. The tetrahedral rotation angle, alpha , is affected by the intercalation of TMP relative to tetramethylammonium (TMA), thus indicating that 2:1 layers are not simply rigid substrates, and that dynamic interactions occur during reactions involving adsorption and exchange.