MesB(C6F5)2 (1) has been prepared from MesMgBr and FB(C6F5)2·OEt2, while Mes2B(C6F5) (2) isreadily available from CuC6F5 and Mes2BBr. The reduction potential Ef"> of 1 vs Cp2Fe0/+ in THF is-1.72 V, while that of 2 is -2.10 V, and that of Mes3B (3) is -2.73 V. 11B and 1H NMR show thatneither 1 nor 2 binds THF significantly. These results have been used to estimate the reduction potentialof B(C6F5)3 in THF as -1.17 V vs Cp2Fe0/+ or as -0.64 V vs SCE.