Conjugates of general formula 1:[A--O--W--Z].sub.a --T 1wherein the moiety A--O-- is the residue of drug of formula A--O--H in which --O--H is a primary or secondary hydroxyl group; a is an integer of from 1 to 30; W is a group of general formula 2: ##STR1## wherein b is an integer of from 1 to 4, B represents a C.sub.1 -C.sub.3 alkylene group and R.sub.1 and R.sub.2 each independently represent hydrogen, halogen, alkyl, phenyl or substituted phenyl; Z is a spacer group and T is a carrier moiety.