Abstract: Suspensions containing 3 percent sodium bentonite (Wyoming) and each of the four methylamine hydrochlorides were prepared so that the latter were present in amounts equal to 25, 50, 75, 100, 125, 200, 400, 600, 800, and 1000 percent of the exchange capacity of the clay. The flocculated clay was removed by decantation, and an oscillating-heating X-ray diffraction (Cu Kα) pattern was made of the first-order diffraction maximum. With these patterns, the thermal stability, mixed layering, and multilayering of the amine-bentonite complex were followed. For methylamine, the data suggest that the first cations to enter lie in the holes with their C-N axes as close to the oxygen surface as possible. As additional methylamine hydrochloride enters, the C-N axis stands perpendicular in the holes to accommodate the chloride, and after all the holes are filled a second layer forms. Although the geometry of the dimethylamine cation permits it to occupy only two out of three holes, it also forms a second layer. The trimethylamine and tetramethyl ammonium cations are so large that they can occupy only one hole in three; this distribution satisfies the cation-exchange capacity, but a second layer does not form.