Inherent oxygen in solid fuels is various in content and occurrence mode related to other elements. Three samples with different inherent oxygen contents were pyrolyzed in an externally heated fixed-bed reactor to characterize their oxygen transformation among solid, water, and gas products in addition to understanding their pyrolysis performances. It was found that the oxygen retained in the solid char decreased with increasing temperature. About 30-50% of the inherent oxygen has transferred into water after pyrolysis at 800 degrees C. The major oxygen-containing gaseous product was CO for the low-oxygen fuel, such as Fugu bituminous coal (FG), whereas this was mainly CO2 for the oxygen-rich fuels, such as Shengli lignite (SL) and herb residue (HR). Fourier transform infrared (FTIR) spectra were taken to analyze the evolution of five oxygen-containing functional groups. The C=O bond existing in ester and carboxyl/carbonyl groups is easy to decompose and is nearly eliminated at 600 degrees C. The hydroxyl group was formed at a high temperature after decomposition at a low temperature, while the C-O single bond in the ether group increased because of the cleavage of C=O bonds.