Disclosed are contrast agents of the formula ##STR1## contained in aqueous compositions and methods for their use in diagnostic radiology of the gastrointestinal tract wherein ##STR2## Z=H, halo, C.sub.1 -C.sub.20 alkyl, cycloalkyl, lower alkoxy, cyano, where the alkyl and cycloalkyl groups can be substituted with halogen or halo-lower-alkyl groups;R=C.sub.1 -C.sub.25 alkyl, cycloalkyl, aryl, or halo-lower-alkyl, optionally substituted with halo, fluoro-lower-alkyl, lower-alkoxy, hydroxy, carboxy or lower-alkoxy carbonyl, lower-alkenyl, lower-alkynyl, lower-alkylene, lower-alkoxy-carbonyloxy;n=1-5;y=0-4; andw=1-4in an aqueous pharmaceutically acceptable carrier.