Trimethylphosphine-pentaborane(9) combined with H- to form a new, stable polyborane complex anion, decahydro(trimethylphosphine)pentaborate(1-) [B5H10.P(CH3)3-]. The potassium and sodium salts of the anion could be isolated as white solids at room temperature. The anion also formed when B5H9.P(CH3)3 was treated with KOH(s) or KOCH3(s). The reaction of the anion with HCl in tetrahydrofuran gave 1-trimethylphosphine-pentaborane(11) [1-P(CH3)3.B5H11] as an unstable intermediate which eventually converted into B4H8.P(CH3)3 and BH2Cl.THF.