he 5f/sup 1/ to 6d/sup 1/ absorption spectrum of /sup 231/Pa/sup 4+/ diluted in a single crystal of Cs/sub 2/ZrCl/sub 6/ has been measured at 4.2 K. Three bands corresponding to the 6d( Gamma /sub 8g/, Gamma /sub 7g/ and Gamma '/sub 8g/) levels are assigned. Extensive vibronic structure has been observed for the lowest 5f to 6d transition and this structure is compared to that reported earlier for the 6d/sup 1/ to 5f/sup 1/ emission spectra in the same system.