The invention relates to compounds of formula (I): ##STR1## wherein R.sup.1, R.sup.2 and R.sup.3 are independently selected from: (i) saturated or unsaturated fatty acyl groups, preferably having from 14 to 24 carbon atoms, more preferably of a type found in nature; or (ii) groups of formula (II): ##STR2## wherein R.sup.4 and R.sup.5 are independently selected from hydrogen atoms, C.sub.1-4 alkyl groups and aryl groups, preferably C.sub.6-10 aryl groups; n represents the integers 0 or 1; A represents either a chemical bond or a straight or branched C.sub.1-6 alkylene chain and R.sup.6 is a triiodinated phenyl group which may also carry other substituents. The invention also relates to X-ray contrast media containing these compounds, as well as methods of enhancing X-ray images of a human subject wherein the contrast media are administered to the subject prior to imaging.