Polycyclic aromatic hydrocarbons (PAHs) are a family of contaminants originating from the incomplete combustion of organic compounds, present in the urban air, water, soil and also foodstuff. Due to their recognised carcinogenicity, their concentration in urban air is routinely monitored and governmental instances have set rules as regards their maximum allowable concentration. Benzo[a]pyrene (BaP) is one of the PAHs, highly toxic, whose concentration correlates well with the total PAHs contents in environmental samples; its detection by immunosensors has been rarely described because of its low molecular weight and poor solubility in water. In this paper, we report the design and testing of optical and piezoelectric immunosensors for the determination of BaP by using Polarisation-Modulation Infrared Reflection-Absorption Spectroscopy (PM-IRRAS) or Quartz Crystal Microbalance with dissipation measurements (QCM-D) as transduction techniques, and a comparison of their analytical performances. We found relevant to set up immunosensors based on a direct or indirect competitive format. BaP was detected by PM-IRRAS with both immunosensor formats with similar sensitivity, ca. 5 mu M. Flow-injection piezoelectric transduction in the indirect competitive format set up led to a slightly better sensitivity than that reached by PM-IRRAS. No general conclusion about the most efficient set up could be drawn from comparing the sensitivities, but the reported data illustrate the potential of PM-IRRAS as well as flow-injection QCM-D as efficient label-free transduction techniques. (C) 2010 Elsevier B.V. All rights reserved.