e biodegradation – ranging from levels 2 to 5 on the [Peters, K.E., Moldowan, J.M., 1993. The BiomarkerGuide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs,NJ, p. 363] scale (abbreviated as _PM level_) -while the shallower Es1 column has undergone more severe biodegradation,ranging from PM level 5 to 8. Both columns show excellent vertical biodegradation gradients, with degree of biodegradationincreasing with increasing depth toward the oil–water contact (OWC). The compositional gradients in theoil columns imply mass transport control on degradation rates, with degradation occurring primarily at the OWC. Thediffusion of hydrocarbons to the OWC zone will be the ultimate control on the maximum degradation rate. The chemicalcomposition and physical properties of the reservoired oils, and the _degradation sequence_ of chemical componentsare determined by mixing of fresh oil with biodegraded oil.The PAH concentrations and molecular distributions in the reservoired oils from these biodegraded columns showsystematic changes with increasing degree of biodegradation. The C3+-alkylbenzenes are the first compounds to bedepleted in the aromatic fraction. Concentrations of the C0-5-alkylnaphthalenes and the C0-3-alkylphenanthrenesdecrease markedly during PM levels 3–5, while significant isomer variations occur at more advanced stages of biodegradation(>PM level 4). The degree of alkylation is a critical factor controlling the rate of biodegradation; in most cases the rate decreaseswith increasing number of alkyl substituents. However, we have observed that C3-naphthalenes concentrations decrease faster than those of C2-naphthalenes, and methylphenanthrenes concentrations decrease faster than that of phenanthrene.Demethylation of a substituted compound is inferred as a possible reaction in the biodegradation process.Differential degradation of specific alkylated isomers was observed in our sample set. The relative susceptibility ofthe individual dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, pentamethylnaphthalene, methylphenanthrene,dimethylphenanthrene and trimethylphenanthrene isomers to biodegradation was determined. The C20and C21 short side-chained triaromatic steroid hydrocarbons are degraded more readily than their C26-28 long sidechainedcounterparts. The C21-22-monoaromatic steroid hydrocarbons (MAS) appear to be more resistant to biodegradationthan the C27-29-MAS.Interestingly, the most thermally stable PAH isomers are more susceptible to biodegradation than less thermally stableisomers, suggesting that selectivity during biodegradation is not solely controlled by thermodynamic stability and thatsusceptibility to biodegradation may be related to stereochemical structure. Many commonly used aromatic hydrocarbonmaturity parameters are no longer valid after biodegradation toPMlevel 4 although some ratios change later than others.The distribution of PAHs coupled with knowledge of their biodegradation characteristics constitutes a useful probe forthe study of biodegradation processes and can provide insight into the mechanisms of biodegradation of reservoired oil.