The invention provides low viscosity iodinated aryl compounds, useful as X-ray contrast agents, of formula (I), wherein n is 0 or 1, and where n is 1 each C6R5 moiety may be the same or different; each group R is a hydrogen atom, an iodine atom or a hydrophilic moiety M or M1, two or three non-adjacent R groups in each C6R5 moiety being iodine and at least one, and preferably two or three, R groups in each C6R5 moiety being M or M1 moieties; X denotes a bond or a group providing a 1 to 7 atom chain linking two C6R5 moieties or, where n is 0, X denotes a group R; each M independently is a non-ionic hydrophilic moiety; and each M1 independently represents a C1-4alkyl group substituted by at least one hydroxyl group and optionally linked to the phenyl ring via a carbonyl, sulphone or sulphoxide group, at least one R group being an M1 moiety; with the proviso that where n is zero either at least one M1 group other than a hydroxymethyl or 1,2-dihydroxyethyl group is present or then if one hydroxymethyl or 1,2-dihydroxyethyl M1 group is present at least one nitrogen-attached or hydroxylated-C3-4 alkyl moiety-containing M group is also present and isomers thereof.