Dimesitylborane dimer has been shown to exist in equilibrium with dimesitylborane monomer in solution. This equilibrium has been investigated by variable concentration and variable temperature multinuclear NMR spectroscopy and values for the dissociation constant, enthalpy and entropy of dissociation were found to be Kdiss=(3.2±0.4)×10−3 M, ΔH=70 kJ mol−1, and ΔS=212 J K−1mol−1, respectively. Ab initio methods have been used to investigate the gas-phase structures and energies of both monomer and dimer, and calculated 11B-NMR shifts are also presented. The solid-state structure of dimesitylborane dimer has been investigated by single crystal X-ray diffraction at 100 K and the position of the bridging hydrogen atoms (BH=1.340(2), 1.342(2) Å, HBH=92.46(14)°) has been determined accurately, for the first time, by single crystal neutron diffraction at 20 K.