The electrochemical behavior of boron-doped diamond films has been investigated in a number of neutral and alkaline solutions with and without nitrate ions. Two kinds of diamond electrode were studied : self-supported film (100 mu m) (sample A) and diamond film (10 mu m) supported on a Si substrate (sample B). It was found that water oxidation and reduction appear at much larger polarizations for diamond electrodes, as compared with platinum and platinized platinum electrodes. In particular, the higher (cathodic) overpotential for hydrogen reduction permits efficient nitrate reduction to ammonia. The underlying Si substrate is shown to take part in the electrochemistry of the diamond electrodes. In the case of the Si-supported electrode (sample B) the reaction with the Si substrate was imminent. For the free-standing diamond electrode (sample A) various impurities in the grain boundaries and at the back of the electrode, including the back metallic contact, intervened with the electrochemistry of the diamond electrode, but to a much lesser extent than with sample B. Meticulous cleaning and careful working practices permitted this interference to be excluded altogether in sample A.