Simulated spectra of the Balmer Da and Ta lines from a Penning discharge in a D-T mixture have been used to assess the potential of the technique in determining the isotopic ratio in the JET divertor neutral gas. Results from measurements in a D-H mixture, with a spectrometer and CCD camera as detector, were used as starting point. The results of repeated multi-Gaussian fits to synthetic spectra, to which random noise had been added, were analysed to evaluate the errors in determining the T/D ratio, for a range of tritium concentrations. It is concluded that T/D ratios of 0.5% can be measured, with an uncertainty of ~ +/- 50% and time resolution 3s, whilst for a 1:1 D-T mixture an uncertainty of +/- 10% is achievable with time resolution ~100 ms. The equipment to make this measurement during JET D-T operation has been installed.