A novel method for the chemical fingerprinting of oil and petroleum products has been developed. The method is based on full-scan mass spectrometry, using a single quadrupole liquid chromatography-mass spectroscopy (LC-MS) instrument, but with direct injection (no chromatographic separation) and without fragmentation of the molecules. In the present work, positive electrospray ionization has been used (ESI-MS). Pattern recognition of the spectra is performed using multivariate data analysis (chemometrics). The mass spectrometer is operated in the scan mode, in the mass number (m/z) range of 65-1000, and gives spectra with one distinct line per integer mass number. One average spectrum is obtained from each analysis. Oils, even the most complex heavy crude oils, can be analyzed directly without pretreatment, except dissolution in dichloromethane. One analysis requires 1 min to perform. Principal component analysis is used to evaluate similarities and differences between oil samples. The projections to latent structures method is used to correlate chemical fingerprints with observed properties. The method has successfully been applied to distinguish between different crude oils and mixtures of crude oils, and it seems to be very promising for a variety of other applications.