Aimed towards the development and effective delivery of highly boronated antitumour agents for use in the neutron capture therapy of cancer, closo-carboranyl derivatives of Nile blue and salts of dequalinium, rhodamine-123 and tetraphenyl phosphonium incorporating nido-carborane counterions have been synthesized and characterized. A preliminary in vitro evaluation in human cell lines indicated the propensity of these agents to target tumour cells, and to deliver therapeutically relevant quantities of boron.