Abstract: The invention concerns gadolinium complexes of formula: ##STR1## in which R represents a group of formula: ##STR2## in which X is Br or I; R.sub.1 is H or optionally hydroxylated alkyl;R.sub.2 is hydroxylated alkyl; oralternatively R.sub.1 is H and R.sub.2 is a group of formula: ##STR3## X being as defined above, and R'.sub.1 and R'.sub.2 being as defined for R.sub.1 and R.sub.2 excepted that they do not represent A, provided that --CO--NR.sub.1 R.sub.2 or --CO--NR'.sub.1 R'.sub.2 comprises at least 2 hydroxyl groups, and M represents H or the cation of an inorganic or organic base.