In the reaction of magnesium with trimethylchlorosilane and t-butyldichlorophosphane the hitherto unknown tri-t-butyl-cyclotriphosphane, (t-BuP)3 (1), is produced besides the partially silylated phosphanes t-BuP(SiMe3)2 and (t-BuPSiMe3)2 as well as the cyclotetraphosphane (t-BuP)4. Shortly after the beginning of the reaction the amount of 1 is about 50 mole-% and decreases during the reaction time. Thermally and in solution the P3 ring compound (1) is surprisingly stable and can be isolated in a pure state by fractional distillation. (t-BuP)3 clearly differs from the oligomeric (t-BuP)4 by the lower melting point, the molar mass and by the 31P NMR and IR spectra. The t-butyl groups in 1 are situated above and below the P3 ring plane.