The adducts pyr•BF2Br and pyr•BFBr2(pyr = pyridine) form fluoroboron cations by displacement of Br−by excess pyridine, the ease of cation formation being pyr2BF2+» pyr2BFBr+» pyr3BF2+•Cl−can be displaced from pyr•BF2Cl and pyr•BFCl2, but much less readily, to form pyr2BF2+, pyr2BFCl+, and, under forcing conditions, a few percent of pyr3BF2+. Non-fluorine-containing mixed boron trihalide adducts of pyridine also form haloboron cations by heaviest-halide-ion displacement, for example pyr•BClI2giving pyr2BClI+, the ease of displacement always being I− > Br− > Cl−, and displacement always occurring more readily from mixed boron trihalide adducts than from unmixed-halogen adducts. The mechanistic implications of this are discussed.orthoSubstituents greatly reduce the ability of pyridine to displace heavy halide ion, so 2-methylpyridine gives 2-Mepyr2BF2+and 2-Mepyr2BFBr+but not 2-Mepyr2BFCl+or 2-Mepyr3BF2+, while 2,6-dimethylpyridine does not form any haloboron cations.19F spin-lattice relaxation times of the fluoroboron cations are much shorter than those of neutral boron trihalide adducts in the same solution, and provide a further diagnostic test for their presence.Keywords: fluoroboron cations, pyridines, mixed boron trihalide adducts, fluorine-19 NMR, boron-11 NMR.