An antioxidant for liquid hydrocarbons such as fuels has the structure ##STR1## wherein X is N or S; R is H or a C.sub.1 to C.sub.5 alkyl moiety; R.sup.1 and R.sup.2 are each independently C.sub.5 to C.sub.18 alkyl or cycloalkyl moieties (which may optionally contain a hetero atom such as oxygen), only one of R.sup.1 and R.sup.2 being present if X is S, or may be combined to form a 5 or preferably 6 membered ring; each R.sup.6 is independently the residue of a C.sub.1 to C.sub.5 aldehyde; and R.sup.7 is the residue of a polyamine of the formula ##STR2## wherein R.sup.3 is a C.sub.8 to C.sub.50 alkyl moiety (which may optionally contain one or more ether linkages); R.sup.4 and R.sup.5 are each independently C.sub.2 to C.sub.6 alkylene moieties; a is 0 or 1; and m is at least 3.