Crosslinked random boron/nitrogen preceramic polymers are prepared by reacting (a) admixture of a trihalogenoborane A and a cyclic boron/nitrogen compound B containing recurring structural units of the following general formula (1): ##STR1## in which A is a halogen atom and R is a hydrogen atom, or a hydrocarbyl, organosilyl or hydrogenoorganosilyl radical, with (b) an amino compound C containing at least one --NH.sub.2 group; the resulting polymers are facilely pyrolyzed into boron nitride ceramic materials.