Abstract: Ammonium-saponite is hydrothermally grown at temperatures below 300°C from a gel with an overall composition corresponding to (NH4)0.6Mg3Al0.6Si3.4O10(OH)2. Using 27Al and 29Si solid-state Magic Angle Spinning NMR techniques it is demonstrated that synthetic ammonium-saponites have a rather constant Si/AlIV ratio (≈5.5) and an AlIV/AlVI ratio that varies between 1.5 and 3.8. The above ratios are independent of the synthesis temperature, although an increasing amount of Si, N, and, to a lesser extent, Al are incorporated in an amorphous phase with increasing temperature. 27Al MAS-NMR is unable to differentiate between Al at octahedral and Al3+ at interlayer sites. CEC, XRD, and the inability to swell prove the AlVI to be mainly on the interlayer sites. Based on the NH4- exchange capacity, X-ray fluorescence, 27Al and 29Si MAS-NMR, it is possible to calculate a relatively accurate structural formula.