The paper describes the measurement of the hydrogen isotope relative concentration by means of the analysis of the emitted flux of neutral particles. The measured flux ratio must be corrected in order to retrieve the proper density ratio, and an approximate conversion function has been derived. A model for the neutral particle emission is described and the results compared with the experiment for ohmic plasma conditions. The H/D behaviour during part of the 1994-1995 campaign is shown. The analysis of the ELM-free phase of hot-ion mode plasmas shows a large increase of the H/D flux ratio which is quenched by the occurrence of a giant Elm. A modified version of the model has been applied to this case in order to assess if the halo neutral density can explain the observation.