A monoazo compound represented by the following formula in the free acid form: ##STR1## wherein D is unsubstituted or substituted phenylene or naphthylene, X is unsubstituted or substituted pyridinio, Z is --SO.sub.2 CH.dbd.CH.sub.2 or --SO.sub.2 CH.sub.2 CH.sub.2 L in which L is a group capable of being split by the action of an alkali, and Y is --V--R in which V is --O-- or --S--, and R is hydrogen or which may be substituted alkyl, cyclohexyl, phenyl which may be substituted, naphthyl which may be substituted or benzyl which may be substituted, or ##STR2## in which R.sub.1 and R.sub.2 independently of one another are each hydrogen or unsubstituted or substituted alkyl, phenyl, naphthyl or benzyl, which is superior in solubility and useful for dyeing or printing fiber materials such as cellulose to a red color with superior build-up properties.