The detailed chemical changes in bitumen brought about over a one year period by an in situ combustion process in an oil sands reservoir have been investigated. The analytical data for the distillation cuts and chromatographic fractions from a core sample and a number of produced oil samples revealed significant changes as a result of the recovery process, particularly for the naphtha and middle distillate cuts. The residue cut did not exhibit any increase in heteroatom or asphaltene content. None of the changes to the produced oil samples would be expected to negatively impact on the subsequent upgrading process. The relationship between the observed changes to the fractionated oil samples and the dynamics of the in situ combustion process are discussed.