The Mg-vermiculite from Santa Olalla has been treated with aliphatic amides--formamide (FM), acetamide (AM) and propionamide (PM)--in aqueous solution. These treatments produce the transformation towards NH 4 -vermiculite and interstratified NH 4 -vermiculite-Mg-vermiculite phases. The NH 4 -vermiculite, Mg-vermiculite and interstratified (mixed-layer) phases have been identified from basal X-ray diffraction (XRD) interval peaks between 10.3 Aa and 14.4 Aa, and confirmed by direct Fourier transform method, as well as by atomic absorption spectrometry (AAS), infrared (IR) spectroscopy and thermal analysis. According to their NH 4 -vermiculite/Mg-vermiculite probability coefficients ratio (P A /P B ), and P AA , these interstratified phases can be divided into 3 categories: 1) If the P A /P B ratio is > or =7/3 and P AA > or =0.7, there are interstratified phases with a strong tendency toward segregation (case of FM, AM and PM). 2) If the P A /P B ratio is between 5/5 and 6/4, with P AA in the range 0.45-0.6, there are nearly regular alternating and random interstratified phases (case of AM and PM). 3) If the P A /P B ratio is > or =5/5 and P AA > or =0.45, there are interstratified phases with a strong tendency toward alternation (case of PM). Experimental evidence reported in the present work indicates that the mechanism of interaction of Mg-vermiculite with FM, AM and PM in an aqueous medium takes place by ion exchange of NH 4 between the layers. The hydrolysis of these aliphatic amides leads to the liberation of NH 4 (super +) into the medium. It has been found that the NH 4 (super +) sorption depends on the physico-chemical characteristics of the particular aliphatic amide, and the transformation of Mg-vermiculite to interstratified and/or NH 4 -vermiculite phases depends on the amide concentration. These treatments allow one to control the formation of interstratified and NH 4 -vermiculite phases.