Defines a method based on inductively coupled plasma optical emission spectrometry (ICP OES) for the determination of manganese content present as methylcyclopentadienyl manganese tricarbonyl (MMT) and of iron content present as ferocene, each from about 0,5 mg/l to about 7,0 mg/l in distillate fuels including those containing up to about 10% (V/V) fatty acid methylester (FAME).