A ceramic composition of matter which is sinterable at low temperatures is composed ofa) 40 to 90% by weight of a finely disperse, high-melting oxidic material andb) 10 to 60% by weight of a finely disperse glass having a softening point of 500.degree. to 850.degree. C., a coefficient of thermal expansion (=CTE) of 2 to 5.times.10.sup.-6 K.sup.-1 (measured in the interval from 0.degree. to 400.degree. C.) and a dielectric constant of less than 5 (measured at 10.sup.6 Hz and 20.degree. C.). The finely disperse oxidic material is composed of a mixture of MgO, Al.sub.2 O.sub.3 and SiO.sub.2 in an Al.sub.2 O.sub.3 /MgO weight ratio of 1.8-4 with an SiO.sub.2 weight fraction of 42-65% by weight or an equivalent quantity of corresponding oxide precursors which, on heating, decompose to give these oxides. The MgO or the MgO precursor has a mean grain size of 0.05 to 3 .mu.m and the SiO.sub.2 or the SiO.sub.2 precursor has a mean grain size of 2 to 20 .mu.m. The composition sinters at 900.degree.-1100.degree. C. and is suitable for producing substrates or metallized housings.