A novel method for "chemical fingerprinting", originally developed for oil and petroleum products, has been applied to biodiesel. The method is based on electrospray mass spectrometry (ESI-MS), efficient data processing, and multivariate data analysis (chemometrics). The method was applied on biodiesel from rapeseed (three different), palm, soybean, used frying, recycled vegetable, and salmon oil (two different), sheep tallow, and animal fat. In addition, dilution series were prepared with rapeseed or salmon biodiesel in petrodiesel in concentrations ranging from 0.5% to 10%. It is demonstrated that ESI-MS and chemometrics can be used to discriminate between biodiesel from different feedstocks and manufacturers, to identify fatty acid methyl esters (FAME) and free fatty acids, and to identify and quantify blend composition. Positive and negative ionization have been used complementarily to specifically identify FAME or free fatty acids, respectively.