Three new formulations of TmDOTP5- (DOTP8- = 1,4,7,10-tetraazacyclododecane-1,4,7,11-tetrakis(methylene-phosphonate)) have been prepared in an effort to develop a low-osmolality form of the Na-23 frequency shift reagent(SR). Equally concentrated (0.32 M) solutions of (MegH)(4)HTmDOTP (Meg = N-methylglucamine or meglumine), Na4HTmDOTP, and [Co(en)(3)](4/3)HTmDOTP have solution osmolalities of 1245, 1040, and 707 mOsm/kg, respectively, comparable to the ionic and non-ionic gadolinium-based MRI contrast agent preparations in clinical use. An analysis of Na-23 and Co-59 frequency shifts induced by TmDOTP5- indicated that Co(en)(3)(3+) can form both. 1:1 and 2:1 adducts with TmDOTP5- with (log) binding constants of 3.1 +/- 0.4 and 2.5 +/- 0.4, respectively. These values were comparable with those obtained by analysis of the H-1 frequency shifts observed for Co(en)(3)(3+) upon binding to HoDOTP5-. The H-1 shifts of Co(en)(3)(3+) signals induced by YbDOTP5- at pH 7.4 were fitted best by a 1:1 binding model with a conditional binding constant of 3.1 +/- 0.2. The Co-59 and H-1 limiting frequency shifts of Co(en)(3)(3+) could be fitted with a dipolar shift model in which the Co atom of the Co(en)(3)(3+) cation is located 5.0 +/- 0.3 Angstrom from the Ln atom of the LnDOTP(5-) chelate, and with an angle of 40 +/- 0.2 degrees between the Co-Ln vector and the 4-fold symmetry axis of the LnDOTP(5-) complex. Ion pairing of Co(en)(3)(3+) and TmDOTP5- was significant enough in both saline and human blood plasma to reduce the effectiveness of the Na-23 frequency SR. Comparisons between all formulations suggested that Na4HTmDOTP represents the best compromise of lower osmolality with minimal reduction of SR shift potency.