A distillate fuel, particularly one which has a high acid number initially or which develops a high acid number as a result of fuel degradation, stabilized with a diaminomethane of the formula: ##STR1## wherein R.sub.1, R.sub.2, R.sub.3 and R.sub.4 may be independently a saturated or unsaturated hydrocarbon group, e.g., alkyl, aryl, aralkyl, alkaryl, cycloalkyl, alkenyl, aralkenyl, alkenylaryl, cycloalkenyl and the like or heterocyclyl groups and in which R.sub.1 and R.sub.2 and/or R.sub.3 and R.sub.4 may be joined together to form a five or six member heterocyclyl ring and R.sub.5 may be hydrogen or lower alkyl is disclosed. A method of stabilizing a fuel oil wherein a stabilizing amount of the above-mentioned diaminomethane is incorporated into the fuel oil is also disclosed.