Abstract: The invention provides iodinated bis phenyl compounds, useful as X-ray contrast agents, of formula I ##STR1## (wherein each C.sub.6 R.sub.5 moeity may be the same or different; each R denotes a hydrogen or iodine atom or a group M, two or three non-adjacent R groups on each C.sub.6 R.sub.5 moiety denoting iodine atoms and one, two or three R groups on each C.sub.6 R.sub.5 moiety denoting M groups;X denotes a group providing a 1, 2 or 3 atom chain linking the two C.sub.6 R.sub.5 groups, preferably where X is or contains in the bridging chain a carbonyloxy group each C.sub.6 R.sub.5 group being a triodophenyl group or a group in which each R is other than hydrogen; and each M is independently a non-ionic hydrophilic moiety, M preferably being a non-ionic hydrophilic moiety comprising a monohydroxy- or polyhydroxy-alkyl group) and isomers, especially stereoisomers and rotamers, thereof.