The kinetic bioconcentration of N-heterocyclic aromatic hydrocarbons and polycyclic aromatic hydrocarbons in mussels (Mytilus galloprovincialis) after short waterborne exposure was studied. Benzo[a]pyrene (BaP), its analogue azaarene 10-azabenzo[a]pyrene (AzaBaP), and their mixture (Mix), were selected to monitor the changes in water concentrations over three days. Decay of both PAHs concentrations in water after 24 h of waterborne exposure to mussels at levels of 10 and 100 μg/L follows a first order kinetic with half-lives of 4–5 h, with residual levels of PAHs below 7%. While steady-state scenarios are well studied, there is a lack of information of what happens under non-steady-state conditions, the main purpose of our paper. A synergistic bioconcentration of the mixture was found (around 800 in the mix vs. around 400 for individual PAHs at 100 μg/L of waterborne exposure). It could be explained by the following reasons. The most polar AzaBaP does not compete with the most non-polar BaP for the same tissue compartments. Whereas BaP aggregate in hydrophobic areas, AzaBaP can also do in hydrophilic areas. Moreover, a chance for complex formation between them by charge-transfer stabilization mechanisms could make possible a higher bioaccumulation as a mixture. Instead, toxicological results suggest an additive behaviour in the mixture performance, dominated by BaP, which is the key PAH controlling phase I metabolization in mussels, since is approx. three times more toxic. These experiments provide useful indications for a rapid assessment of PAHs kinetic bioconcentration in mussels.