An objective of the present invention is to provide a phosphorous-containing siliceous material having a specific permittivity of not more than 3.5. The phosphorus-containing silazane composition according to the present invention is characterized by comprising a polyalkylsilazane and at least one phosphorus compound in an organic solvent. A phosphorus-containing siliceous film may be formed by coating the composition onto a substrate to form a film which is then prebaked at a temperature of 50 to 300.degree. C. and is then baked in an inert atmosphere at a temperature of 300 to 700.degree. C. The phosphorus compound according to the present invention is preferably a pentavalent phosphoric ester or phosphazene compound.