The use of a chemically inert and essentially biocompatible material for cellular biosensing is an attractive idea. In this context we have studied the operation of diamond-based ion-sensitive field effect transistors (ISFETs) within solutions of varying pH, and alkali-halide concentrations. In particular, we report the use of an inexpensive diamond substrate material, often referred to as ‘black’ diamond. pH sensitivity was observed when devices based on hydrogen-terminated p-type surfaces where employed, provide some surface oxidation was performed prior to their use. Variation in the threshold voltage for ISFET operation of the order of 20mV/pH unit was determined. In terms of operation in potassium iodide solution, we have shown that the device is shows selective sensitivity to the iodide species, despite the equi-molar presence of both K+ and I- species. The origin of this selectivity is discussed.