A silicone resin, curable to a resin of low coefficient of thermal expansion, high glass transition temperature and high modulus, has the empirical formula (R.sub.3SiO.sub.1/2).sub.a(R.sub.2SiO.sub.2/2).sub.b(RSiO.sub.3/2).sub.c(- SiO.sub.4/2).sub.d wherein each R is a hydrocarbon or substituted hydrocarbon group or a hydrogen atom; and a=0.02 to 0.8; b=0 to 0.4; and c+d=0.2 to 0.98, where a+b+c+d=1.0, characterized in that at least 2 mole % of the siloxane units in the resin are of the formula R'SiO.sub.1/2, RR'.sub.2SiO.sub.1/2 or R'.sub.2SiO.sub.2/2, wherein each R' is an alkenyl group.