The sensitivity of diatomic ro-vibronic band emission structure to isotopic composition provides a possibility of detecting the proportions of deuterium and tritium in the tokamak via the fusion relevant molecules BeD/BeT and CD/CT. Studies of the BeD A 2P - X 2S transition at ~4990Å and the CD A 2D - X 2P transition at ~4320Å in the visible spectral region (Duxbury et al, 1997) from the JET tokamak have already shown the sensitivity of the band envelopes to the effective rotational and vibrational temperature.Modelling parameters are not available for all BeH and CH isotopologues. Also distinguishing molecular features in JET tritium experiments are masked by unfavourable band head shifts, overlapping atomic lines and by the low levels of tritium introduced.