Abstract A number of overtone and combination banda in the i.r. spectrum of CD 3I have been measured at medium or high resolution and the rotational fine structure of these bands has been analyzed to obtain estimates of the rotational constants of the excited vibrational states. From these results we are able to estimate the values of the rotational constants A 1, A 2, A 4 and B 4. The new measurements confirm many previous assignments of the overtone and combination bands with some revisions and also reveal five bands not previously reported. Further, the analysis of the rotational structure yields precise estimates of the vibrational origins of the overtone and combination bands and these values have been used to estimate the anharmonic constants x ss' and gtt' for CD 3I.