We report on the realization of a optoacoustic sensor based on mid-infrared quantum-cascade lasers (QCLs) for the detection of nitric oxide (NO) and formaldehyde (CH(2)O). A resonant photoacoustic cell equipped with 4 electret microphones was excited in its first longitudinal mode by the modulated laser light. A detection limit for of 300 parts in 10(9) (ppbv) for NO and 150 ppbv for CH(2)O is found, using distributed feedback QCLs operating in pulsed mode at 5.34 mu m and 5.6 mu m, respectively.