A monoazo compound represented by the general formula (I): ##STR1## wherein A represents alkylene which may be substituted or a group: --(CH.sub.2)m--Q.sup.1 --(CH.sub.2)n-- wherein Q.sup.1 is --O--, --S-- or --NR.sup.3 --, m and n are, independently with each other, 2, 3 or 4 and R.sup.3 is hydrogen, alkyl which may be substituted or phenyl which may be substituted; B represents .beta.-carboxyvinyl, alkyl which may be substituted or phenyl which may be substituted; D represents phenylene which may be substituted; X represents aliphatic, aromatic, alicyclic, cyclic or unsubstituted amino; R.sup.1 and R.sup.2 represent, independently with each other, hydrogen or alkyl which may be substituted; and SO.sub.2 Y is a reactive vinyl sulfone group; or a salt thereof, and a process for dyeing or printing fiber materials using said compound. Said process enables red dyeing or printing of hydroxyl group and/or amido group containing organic materials with excellent fixing in deep dyeing.