Polycyclic aromatic hydrocarbons (PAHs) are environmentally significant because of their ubiquity and the toxicity of some. Their recalcitrance and persistence makes them problematic environmental contaminants. Microbial degradation is considered to be the primary mechanism of PAH removal from the environment. Biodegradation kinetics of individual PAHs by pure and mixed cultures have been reported by several researchers. However, contaminated sites commonly have complex mixtures of PAHs whose individual biodegradability may be altered in mixtures. Biodegradation kinetics for fluorene, naphthalene, 1,5-dimethylnaphthalene and 1- methylfluorene were evaluated in sole substrate systems, binary and ternary systems using Sphingomonas paucimobilis EPA505. The Monod model was fitted to the data from the sole substrate experiments to yield biokinetic parameters, (qmax and Ks). The first order rate constants (qmax/Ks) for fluorene, naphthalene and 1,5- dimethylnaphthalene were comparable, although statistically different. However, affinity constants for the three compounds were not comparable. Binary and ternary experiments indicated that the presence of another PAH retards the biodegradation of the co-occurring PAH. Antagonistic interactions between substrates were evident in the form of competitive inhibition, demonstrated mathematically by the Monod multisubstrate model. This model appropriately predicted the biodegradation kinetics in mixtures using the sole substrate parameters, validating the hypothesis of common enzyme systems. Competitive inhibition became pronounced under conditions of: Ks1 << Ks, S1 >> Ks1 and S1 >> S. Experiments with equitable concentrations of substrates demonstrated the effect of concentration on competitive inhibition. Ternary experiments with naphthalene, 1,5-dimethylnapthalene and 1-methylfluorene revealed preferential degradation, where depletion of naphthalene and 1,5-dimethylnapthalene proceeded only after the complete removal of 1-methylfluorene. The substrate interactions observed in binary and ternary mixtures require a multisubstrate model to account for simultaneous degradation of substrates. However, developing models that account for sequential degradation may be useful in scenarios where PAHs may not be competitive substrates. These mixture results prove that substrate interactions must be considered in designing effective bioremediation strategies and that sole substrate performance is limited in predicting biodegradation kinetics of complex mixtures.