The crystal structure of Pr 2 (S 2 O 6 ) 3 (Ph 3 PO) 6 (H 2 O) 6 (1), has been determined and is compared with the previously reported Nd 2 (S 2 O 6 ) 3 (Ph 3 PO) 4 (H 2 O) 8 (2), and Nd 2 (S 2 O 6 ) 3 .14H 2 O (3). The steric congestion in the primary coordination sphere, evaluated by steric angle sum (SAS) calculations, shows that all the complexes have SAS values within the expected range for the coordination number of the metal. The incorporation of extra Ph 3 PO in 1 is accommodated by a very long Pr OH 2 distance. The behaviour of lanthanide dithionates and their triphenylphosphine oxide complexes under electrospray mass spectrometry conditions has been investigated. For Ln 2 (S 2 O 6 ) 3 , the expected ions, Ln(S 2 O 6 ) + and Ln(S 2 O 6 ) 2 - , and their methanol solvates, are observed as weak signals. The relationship between these and the major ions observed in the gas phase is discussed. The spectra of the triphenylphosphine oxide complexes show no lanthanide containing ions.