The invention relates to a process for preparing trisilylamine and polysilazanes in the liquid phase, in which ammonia dissolved in an inert solvent is initially introduced in a substoichiometric amount relative to monochlorosilane which is likewise present in an inert solvent. The reaction is carried out in a reactor in which trisilylamine formed according to the following equation 4NH3+3H3SiCl→3NH4Cl+(SiH3)3N and polysilazanes are formed. The reactor is subsequently depressurized and TSA is separated off in gaseous form from the product mixture. The TSA obtained is purified by filtration and distillation and obtained in high or very high purity. Further ammonia dissolved in an inert solvent is subsequently introduced into the reactor, using, together with the previously introduced amount of ammonia, a stoichiometric excess of ammonia relative to the amount of MCS previously present. Excess ammonia is subsequently discharged, inert gas is introduced and the bottom product mixture from the reactor is passed through a filter unit, with solid ammonium chloride being separated off and a liquid mixture of polysilazanes and solvent being obtained.