The invention relates to compounds of the general formula (I): ##STR1## wherein Im represents an imidazolyl group of the formula: ##STR2## R.sup.1 represents a hydrogen atom or a group selected from C.sub.1-6 alkyl, C.sub.3-6 alkenyl, C.sub.3-10 alkynyl, C.sub.3-7 cycloalkyl, C.sub.3-7 cycloalkylC.sub.1-4 alkyl, phenyl, phenylC.sub.1-3 alkyl, --CO.sub.2 R.sup.5, --COR.sup.5, --CONR.sup.5 R.sup.6 or --SO.sub.2 R.sup.5 ;R.sup.5 and R.sup.6, which may be the same or different, each represents a hydrogen atom, a C.sub.1-6 alkyl or C.sub.3-7 cycloalkyl group, or a phenyl or phenylC.sub.1-4 alkyl group, in which the phenyl group is optionally substituted by one or more C.sub.1-4 alkyl, C.sub.1-4 alkoxy or hydroxy groups or halogen atoms, with the proviso that R.sup.5 does not represent a hydrogen atom when R.sup.1 represents a group --CO.sub.2 R.sup.5 or --SO.sub.2 R.sup.5 ;one of the groups represented by R.sup.2, R.sup.3 and R.sup.4 is a hydrogen atom or a C.sub.1-6 alkyl, C.sub.3-7 cycloalkyl, C.sub.3-6 alkenyl, phenyl or phenylC.sub.1-3 alkyl group, and each of the other two groups, which may be the same or different, represents a hydrogen atom or a C.sub.1-6 alkyl group;Q represents a hydrogen or a halogen atom, or a hydroxy, C.sub.1-4 alkoxy, phenylC.sub.1-3 alkoxy or C.sub.1-6 alkyl group or a group --NR.sup.7 R.sup.8 or --CONR.sup.7 R.sup.8 ;R.sup.7 and R.sup.8, which may be the same or different, each represents a hydrogen atom or a C.sub.1-4 alkyl or C.sub.3-4 alkenyl group, or together with the nitrogen atom to which they are attached form a saturated 5 to 7 membered ring;n represents 1, 2 or 3; andA-B represents the group CH--CH.sub.2 or C.dbd.CH; and physiologically acceptable salts and solvates thereof.The compounds are potent and selective antagonists of the effect of 5-HT at 5-HT.sub.3 receptors and are useful, for example, in the treatment of psychotic disorders, anxiety, and nausea and vomiting.