Combustion chamber deposits from a 1996 GM3800 engine operating on a base fuel or the base fuel containing the organometallic antiknock additive methylcyclopentadienyl manganese tricarbonyl were examined. Scanning Electron Microscopy and Energy Dispersive X-ray spectroscopic analysis were performed to identify the morphology and the bulk chemical composition. Glow-Discharge Mass Spectrometry and X-ray Diffraction analyses were also used to characterize the bulk chemical composition and crystalline structure of the deposits. In addition, X-ray photoemission and X-ray photoabsorption spectra for the deposits were compared to a series of Mn compounds to model and aid quantification of the constituents. Results reveal a mixture of Ca-sulfate, Mn-phosphate and Mn-oxide in the bulk of the deposits and a mixture of Mn-sulfate, Mn-phosphate and Mn-oxide on the surface of the deposits.