The [BX3(EMe2)] (X = Cl, Br, I; E = Se or Te) have been prepared by reaction of BX3 with the EMe2 in hexane under anhydrous conditions. The X-ray crystal structures of [BX3(TeMe2)] (X = Cl, Br, I) and [BX3(SeMe2)] (X = Cl, Br) have been determined; all are pseudo-tetrahedral monomers and show d(B−E) decreases with halogen, Cl > Br > I. Multinuclear NMR data (1H, 11B, 77Se and 125Te) are reported and compared with data on the corresponding [BX3(SMe2)], and the trends discussed. The unstable [BF3(SeMe2)], prepared from BF3 and SeMe2 in the absence of a solvent, has been similarly characterised by multinuclear NMR spectroscopy, and evidence for the existence of unstable [BF3(TeMe2)] obtained for the first time, although it could not be obtained pure. The results are discussed in the light of recent theoretical modelling of boron halide adducts.