Abstract: Field desorption of mass spectrometry, ~1H NMR, and inverse gated decouping of ~(13)C NMR are used to analyze calcium petroleum sulphonates used as detergen-dispersant additive for lubricating oil directly. The analytical results, involving distribution of molecular weight, average molecular weight, structure of the hydrocarbon portion of the sulphonates and so on, are the same as those obtained from a reported method, in which the samples were an alyzed only after hydrolysed to the corresponding hydrocabons. The pre-treament of our method is much easier than before.