The biodegradation patterns of polycyclic aromatic hydrocarbon (PAH) mixtures were studied in systems with and without sediments. Naphthalene showed no change in its behaviour in the presence of any other PAH. In sediment-free systems, acenaphthene showed a diauxic pattern in the presence of naphthalene, but was unaffected with fluoranthene. Diauxy was not observed in the same mixture in sediment-containing systems. Sediment-free experiments with fluoranthene indicated it was the only PAH not degraded when present alone, and it was only degraded with naphthalene. In sediment-containing systems, fluoranthene degradation only occurred above a critical naphthalene concentration. Experiments using all three PAHs in both sediment-free and -containing systems showed results similar to those obtained using pairs. This work indicated that mixtures of PAHs do have different biodegradation patterns to single compounds. These patterns may be additive. Sediments act as a sink for PAHs, highlighting the concentration dependence of the patterns observed.