Details of the reaction of B2H4[P(CH3)(3)](2) With the C(C6H5)(3)(+) cation for the synthesis of the B3H4[P(CH3)(3)](3)(+) cation are described. The reaction was thought to proceed through the formation of the ''B2H3P[(CH3)(3)](2)(+)'' cation followed by the cluster expansion reaction with B2H4[P(CH3)(3)](2). The triborane complex cation was fluxional with respect to the migration of the four borane hydrogen atoms. The B-11, P-31 and H-1 NMR chemical shifts were determined at low and high temperatures. The B-11, P-31, and F-19 NMR data for BF3P(CH3)(3), a byproduct in the above reaction, were also reported.