Interactions between select microorganisms and heavy crude oils in which the microorganisms have been introduced under controlled conditions proceed via a complex set of multiple biochemical and chemical reactions. Extensive studies in this laboratory have shown that such reactions are not random and differ from those involved in biodegradation of oils and follow distinct trends which can be categorized by means of characteristic chemical markers. These markers include mass spectrometric fragmentation patterns of light and heavy hydrocarbons, heterocyclic, organometallic compounds, trace metals, and the contents of heteroatoms. Bioconversion of heavy crude oils depends on the distribution of polar compounds containing nitrogen (N), sulfur (S), oxygen (O), and trace metals. The overall effects of induced biochemical reactions are a significant lowering (24-40%) of the N, S, O, and trace metal contents. Concurrently, there is also a redistribution of hydrocarbons (HC). The reactions are both biocatalyst and crude oil dependent and, in terms of chemical mechanisms, appear to involve the asphaltene and the associated polar fractions. Recent studies dealing with biochemical reactions in heavy crudes will be discussed.