This invention relates to compounds having the following general formula: ##STR1## in which n is an integer from 0 to 4; R.sub.1 and R.sub.2 represent independently from each other a radical of the formula --CONHR.sub.5 in which R.sub.5 is an alkyl radical having 1-4 carbon atoms or a hydroxyalkyl radical having 1-4 carbon atoms or a radical of the formula ##STR2## in which R.sub.6 and R.sub.7 are alkyl radicals having 1-4 carbon atoms; and R.sub.3 and R.sub.4, independently from each other, represent a hydrogen atom or an alkyl radical having 1-4 carbon atoms. Said compounds are useful as X-ray contrast media, particularly for myelography.