A tritiated water vapour monitor that utilizes modulation of the input tritium concentration to allow phase sensitive detection is described. This measurement technique is useful in those measurement situations where there are slowly varying interferences from other radiations or species; or where it is necessary to measure quite low concentrations. The increased immunity to interferences is because, in general, the frequency spectrum of the interferences is peaked at zero frequency and will not have major components at the modulation frequency. The increased sensitivity of the monitor is because it is not limited by input leakage currents nor slow drifts in zero.