KMg2AlSi4O12 is a phyllosiloxide isostructural with phlogopite mica, but totally free of OH- ions. It decomposes at approximate to 950 degrees C at atmospheric pressure but remains stable up to at least 1350 degrees C under high pressures. Its chemical compatibility with alpha-alumina, MgAl2O4 spinel, forsterite, beta-SiC and borosilicate glass selected as representative of fibres and matrices in ceramic matrix composites (CMCs), has been assessed via annealing experiments on multilayers and particulate composites at 900-1200 degrees C. At T = 900 degrees C and P = 100 MPa, the phyllosiloxide is chemically stable with respect to all the ceramics. At higher temperatures, interdiffusion occurs with the formation of various reaction products. At T = 1050 degrees C and P = 2 GPa, the extent of the reaction zone is larger for both alpha-alumina and forsterite than for spinel and beta-SiC, whereas at 1200 degrees C, the reactivity of the phyllosiloxide with all the ceramics becomes about the same. Borosilicate glass with a softening point lower than the decomposition onset of KMg2AlSi4O12 at relatively low pressures seems to be an ideal model matrix material for assessing the potential of the phyllosiloxide as an interphase material in CMCs.