A method was developed for the analysis of high-boiling petroleum saturates by laser desorption and silver cationization (LDI-Ag) coupled to Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS). Commercial polywax standards, petroleum vacuum gas oil (VGO), and vacuum resid (VR) were successfully analyzed with a carbon number span from 30 to 100. Representative model compounds were studied to evaluate relative sensitivity of the ionization method. "Soft" ionizations have been observed for all hydrocarbon types, generating primarily M + Ag+ center dot ions. Because of the existence of silver isotopes, an ultrahigh mass resolution [e.g., resolving power (RP) > 65 000 at mass similar to 1000 Da] is found necessary to resolve the key mass overlap ((AgH2)-Ag-107/Ag-109 doublet; Delta M similar to 16 mDa). Petroleum saturate composition was arranged by homologous series with a general chemical formula (CcH2c+z), where c stands for the carbon number and Z is generally referred as the hydrogen deficiency index. A comparison of Z number distributions between VGO and VR saturates revealed significant differences in the compositions and structures. The Z number of VGO ranges from 2 to 12, corresponding to 0-7 naphthenic rings, respectively. This result is consistent with the prior knowledge on VGO composition from conventional analytical methods. For VR saturates, Z numbers ranging from 2 to -24 (0-13 naphthenic rings) have been observed. Because olefin and aromatic contents in saturate samples are very low, the large Z number (Z less than 12) are attributed to the presence of high-order naphthenic ring structures (>7 ring naphthenes).