Combustion chamber deposits (CCDs) have been the subject of much research aimed at understanding their role in driveabilty and emissions performance in SI engines. Correlations have been drawn between CCDs and octane requirement increase (ORI). In order to gain a better understanding of the complex structural chemistry of these carbonaceous deposits a large number of these deposits, generated in several SI engines, have been investigated by solid-state 13C nuclear magnetic resonance (NMR) spectroscopy. 13C NMR methodologies developed for the study of coal were used to obtain structural and molecular parameters for each of the deposits. These parameters provide an “average molecule” description which defines the average building block of the polymeric backbone of CCDs. These parameters were then correlated with CCD weight and thickness measurements, ORI and cleanliness ratings, and fuel composition. 31P NMR was used to investigate the phosphorus chemistry of oxidized anti-wear agents, originating in the lubricant, which become incorporated into the CCDs. 1H NMR spectral and relaxation data reveals the higher mobility of aliphatic regions of the deposit, with aromatic regions appearing to be motionally restricted, yielding evidence for sample heterogeneity. The overall view that one obtains is that the CCDs are formed via free radical and oxidative product condensation reactions, which polymerize together the heavier aromatic components of the fuel. The degree of polymerization and contraction of the CCD structure towards a “graphite-like” structure is dependent on the fuel used and the severity of the combustive/oxidative environment.