Lithium cationization can significantly extend the compositional range for analysis of petroleum components by positive electrospray ionization [(+) ESI], by accessing species that lack a basic nitrogen atom and, hence, are not seen by conventional (+) ESI that relies on protonation as the primary ionization mechanism. Here, various solvent compositions and lithium salts enabled us to optimize ionization by formation of lithium adducts ([M + Li](+)), and the results are compared to production of [M + H](+) by conventional (+) ESI with formic acid. Lithium cationization (+) ESI Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) of Athabasca bitumen heavy vacuum gas oil (475-500 degrees C) and North and South American crude oils demonstrates considerable improvement over protonation for production of ions from compounds belonging to SxOy (SO, SO2, SO3, SO4, S2O, S2O2, etc.) heteroatom classes. Those compounds exhibit much higher affinity for lithium cation than for proton and yield abundant [M + Li](+) ions. Li+ cationization thus opens a pathway for detection and characterization of SxOy class compounds that preferentially concentrate at the interface in oil/water emulsions.