The preparation of high temperature ceramics simultaneously containing silicon, nitrogen and carbon has only relatively recently become an area of interest for inorganic crystal chemists, and the recent discovery of a new series of carbonitrides with the general formula MM’Si4N6C is of interest because of the good high temperature properties they appear to display. On the one hand, M and M’ can be the same trivalent metal - either rare earth or yttrium; in this case, the resulting compounds display orthorhombic (pseudo-hexagonal) structures. Alternatively the metals may be a mix of di- (Ca,Sr, Ba) and tri-valent (Y,Ln) cations, in which case the carbon is replaced by nitrogen, and the overall symmetry is hexagonal. Other quaternary nitrides of a similar type can be produced if the two metal cations remain trivalent and one of the silicon atoms is replaced by aluminium. The present study describes the preparation of powder samples of Y2Si4N6C and LaYSi4N6C starting from YH2, La, Si3N4 and carbon precursors, and summarises attempts to achieve a dense product by hot-pressing at 1700-1800oC. Some preliminary mechanical property measurements are included.