Raman spectroscopic measurements have been made of aqueous solutions of La(ClO 4 ) 3 , La 2 (SO 4 ) 3 , and Na 2 SO 4 in water and heavy water, in the terahertz frequency region (40-1400 cm
À1) and down to low , the n 1 -SO 4 2À band in La 2 (SO 4 ) 3 (aq) showed systematic differences from that in Na 2 SO 4 (aq). This is consistent with a n 1 -SO 4 2À band at 983.3 cm À1 that can be assigned to the existence of an outer-sphere complex (OSCs). The observed change of the degree of sulfato-complex formation with dilution reflects the stepwise sulfato-complex formation. A K 3 -value has been determined at 0.9 of the equilibrium between OSC and ISC.