A combination of analytical techniques, including X-ray photoelectron spectroscopy (XPS), solid state C-13 nuclear magnetic resonance (NMR) spectroscopy, and supercritical fluid extraction/mass spectrometry (SFE/MS), were used to characterize the detailed composition and structure of coke formed on catalyst in the fluid catalytic cracking (FCC) process. By characterizing coke samples from a series of designed FCC experiments, the effects of conversion on coke composition were systematically studied. SFE is shown to be an effective technique for removing low molecular weight coke molecules from the catalyst. When combined with mass spectrometry, the technique provided molecular level information of the extracted coke species. The coked catalysts were directly analyzed by XPS and NMR to obtain information relevant to surface and bulk coke structures, respectively. The study revealed the presence of two types of nitrogen-based coke and showed that N distributions were strongly affected by FCC conversion level. The study also suggests that most nitrogen-containing coke is formed in the earlier stages of cracking while hydrocarbons are the primary contributors to coke yield in the later stages of cracking. The aromaticity of coke remains fairly constant at high conversions.