The PAH concentrations and molecular distributions in the reservoired oils from these biodegraded columns show systematic changes with increasing degree of biodegradation. The C₃₊-alkylbenzenes are the first compounds to be depleted in the aromatic fraction. Concentrations of the C_0–5-alkylnaphthalenes and the C_0–3-alkylphenanthrenes decrease 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 decreases with 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 of the individual dimethylnaphthalene, trimethylnaphthalene, tetramethylnaphthalene, pentamethylnaphthalene, methylphenanthrene, dimethylphenanthrene and trimethylphenanthrene isomers to biodegradation was determined. The C₂₀ and C₂₁ short side-chained triaromatic steroid hydrocarbons are degraded more readily than their C_26–28 long side-chained counterparts. The C_21–22-monoaromatic steroid hydrocarbons (MAS) appear to be more resistant to biodegradation than the C_27–29-MAS.

Interestingly, the most thermally stable PAH isomers are more susceptible to biodegradation than less thermally stable isomers, suggesting that selectivity during biodegradation is not solely controlled by thermodynamic stability and that susceptibility to biodegradation may be related to stereochemical structure. Many commonly used aromatic hydrocarbon maturity parameters are no longer valid after biodegradation to PM level 4 although some ratios change later than others. The distribution of PAHs coupled with knowledge of their biodegradation characteristics constitutes a useful probe for the study of biodegradation processes and can provide insight into the mechanisms of biodegradation of reservoired oil.