Nitrosyl tetrafluoroborate may be used to diazotise an aniline to make a diazonium tetrafluoroborate salt. The need to isolate this toxic intermediate is avoided by carrying out the reaction in a low boiling solvent (such as dichloromethane or acetonitrile); then, after addition of a higher boiling one (such as dichlorobenzene), distilling off the first with the water formed in the diazotisation. The boron trifluoride generated in the dediazoniation is then reacted with NOF·6HF formed in the reaction of sodium nitrite with HF, to remake the nitrosyl tetrafluoroborate. It was not possible to isolate this compound quantitatively since it is decomposed by both the sodium fluoride and the water also present. The solution in HF is therefore used in the recycle and the HF recovered when the lower boiling solvent is distilled off.