Abstract: Apparatus for, and a method of, simultaneously measuring the concentration of a selected ion species in a solution and the pH of the solution, uses an ion selective electrode, a reference electrode and an ISFET immersed in the solution. The ion concentration is determined in a first circuit from the potential difference between the ion selective electrode and the reference electrode, and the pH is determined in a second circuit from the current flowing between the ISFET and the reference electrode. The reference electrode is connected into the second circuit via a high resistance so as to isolate the two circuits from each other. The earth potential of the solution is connected to provide the earth potential of the first circuit, and is connected to the second circuit via a high capacitance so as to provide a virtual earth thereforee. Each circuit is supplied from a separate power source, and the circuit outputs are supplied via respective isolation amplifiers to a multi-channel meter.