The first-generation contrast agents were all ionic monomers, consisting of a tri-iodinated benzene ring with 2 organic side chains and a carboxyl group. Diatrizoate or iothalamate are common iodinated anions, conjugated with a cation, sodium or meglumine. The ionization at the carboxyl-cation bond makes the agent water soluble.
Ionic monomers have the highest osmolality (high-osmolar contrast media (HOCM) possess an osmolality seven to eight times higher than plasma) of the different groups of contrast agents (CM ratio=1.5) and the lowest viscosity. The osmolality in solutions of ionic monomers ranges from 600 to 2100 mOsm/kg (human plasma = 290 mOsm/kg). These high osmolality is related to some of the adverse reactions. HOCM's have been widely replaced by newer contrast agents with improved tolerability and safety profiles.
Examples of HOCM's are Renografin®-60, Hypaque 76, Hypaque Meglumine, Hypaque Sodium and Conray®.

See also Ionic Contrast Agents.