When aromatic hydrocarbons are present in contaminated soils, they often occur in mixtures. The impact of four different (3-ring) nitrogen-containing polycyclic aromatic hydrocarbons (N-PAHs) on 12/14C-phenanthrene and 12/14C-benzo[a]pyrene (B[a]P) mineralisation in soil was investigated over a 90 d incubation period. The results revealed that both 12/14C-phenanthrene and 12/14C-benzo[a]pyrene showed no significant mineralisation in soils amended with 10 mg kg –1 and 100 mg kg –1 N-PAHs (p>0.05). However, increases in lag-phases and decreases in the rates and extents of mineralisation were observed, over time. Among the N-PAHs, benzo[h]quinoline impacted 14C-phenanthrene mineralisation with extended and diauxic lag phases. Furthermore,12/14C-B[a]P and 14C-benzo[a]pyrene–nitrogen-containing polycyclic aromatic hydrocarbons (14C-B[a]P-N-PAHs) amended soils showed extensive lag phases (> 21 d); with some 14C-B[a]P-N-PAH mineralisation recording <1% in both concentrations (10 mg kg –1 and 100 mg kg –1), over time. This study suggests that the presence of N-PAHs in contaminated soil may impact the microbial degradation of polycyclic aromatic hydrocarbons (PAHs) and the impact was most likely the result of limited success in achieving absolute biodegradation of some PAHs in soil.