The hydrothermal reactivity of a Mg-vermiculite from Santa Olalla (Spain) was studied experimentally at 150 and 300°C, 75 and 100 bars, respectively, in (Na, K, Ca, Mg) chloride and sulphate solutions (liquid/solid ratio = 10). The formation of mixed-layer clays is demonstrated by FT-IR spectroscopy, X-ray diffraction (XRD) and electron microprobe. The gradual decrease of the 3220 cm−1 infrared band intensity, characteristic of free water, is interpretated as the dehydration of the interlayer Mg cation of the vermiculite. The presence of ‘brucite type’ sheets is also observed by the increase of 3555 cm−1 and 3420 cm−1 band intensities. The XRD results show that run products are able to expand when glycolated. Microprobe analyses document the decrease of IVSi content and a significant enrichment in Mg in the run products. The formation of a mixed-layer clay comprising saponite and chlorite layers is proposed.