The structure of [PPh3(benzyl)][B10H11] was determined at -123 degreesC and 24 degreesC by single-crystal X-ray analyses. The B-10 core of [B10H11](-) is similar in shape to that of [B10H10](2-). The 11th H atom asymmetrically caps a polar face of the cluster and shows no tendency for disorder in the solid state. Variable temperature multinuclear NMR studies shed light on the dynamic nature of [B10H11](-) in solution. In addition to the fluxionality of the cluster H atoms, the boron cage is fluxional at moderate temperatures, in contrast to [B10H10](2-). Multiple exchange processes are believed to take place as a function of temperature. Results of ab initio calculations are presented. Crystal data: [PPh3(benzyl)][B10H11] at -123 degreesC, P2(1)/c, a = 9.988(2) Angstrom, b = 18.860(2) Angstrom, c = 15.072(2) Angstrom, beta = 107.916(8)degrees, V 2701.5(7) Angstrom(3), Z = 4; [PPh3(benzyl)][B10H11] at 24 degreesC, P2(1)/c, a = 10.067(5) Angstrom, b = 19.009(9) Angstrom, c = 15.247(7) Angstrom,beta = 107.952(9)degrees, V = 2775(2) Angstrom(3), Z = 4.