We describe a thermal wave imaging system based on gas-microphone detection of the harmonic component of the photoacoustic (PA) signal. In this system the sample is excited by a laser beam modulated at a frequency ƒ and the detection of the associated thermal wave field is carried out at a frequency 2ƒ. The technique demonstrates lateral resolution comparable to conventional thermal wave systems based on detection of the fundamental component of thePAsignal, but provides for better contrast in depth-profiling applications.