Cyclic phosphonitrilic halide trimer is made in high yield and at a rapid rate by adding NH₃ or ammonium halide to a pyridine-hydrogen halide complex and adding phosphorus pentahalide and a solvent for the trimer (e.g. monochlorobenzene) and heating the mixture to above 90°C to form trimer which dissolves in the solvent and then separating the trimer-solvent phase from the residue phase of pyridine-hydrogen halide complex and recovering trimer from the solvent phase and recycling the pyridine-hydrogen halide phase to a subsequent procedure conducted in the same manner. The process can also be adapted to provide linear species in high yield also.