The radical cation P840" was studied in frozen suspensions of Chlorobium limicolu f. sp. thiosulphatophilum membranes using ENDOR and Special TRIPLE spectroscopies. The spectra show that P840" arises from a bacteriochlorophyll a 'special' pair with a highly symmetrical distribution of electron spin density between the constituent bacteriochlorophylls. Special TRIPLE spectroscopy has resolved the separate contributions of the two halves of the pair and revealed small deviations from a 1 : 1 electron spin density distribution. Nevertheless P840" appears to come the closest yet to the symmetrical 'dimer' originally proposed for the structure of the primary donor radical cation (P870") in purple non-sulphur photosynthetic bacteria.