We present a first photothermal mid-IR spectroscopy setup for liquids on the example of sensitive detection of water traces in organic solvents (ethanol and chloroform) and as a most difficult application in aircraft’s jet fuel. Classical absorption spectroscopy based on Lambert-Beer´s law relies on detection of minute differences in light intensity. However, as absorbance does not scale with laser power it does not fully benefit from quantum cascade lasers’ (QCLs) high power and brilliance. [1] At the contrary, photothermal spectroscopy (PTS) is an indirect detection approach where the generated analytical signal is directly proportional to the applied laser power. Increasing the power means that the sensitivity is enhanced, too. In PTS the sample is illuminated by a pulsed mid-IR excitation source causing a periodical heating and cooling of the sample. The resulting photo-induced thermal gradient ΔT can be probed as a consequent refractive index change (Δn) by means of a second laser source, a so called “probe laser”. The challenge lies in detecting the smallest Δn. To do that, we use an interferometric approach. In particular, our latest generation liquid PTS IR sensor consists of a Mach-Zehnder Interferometer (MZI) able to sense sub-nm phase shifts Δφ between its two arms. In detail, we use a HeNe probe laser and an external cavity (EC)-QCL pump laser tuneable from 1730 to 1565 cm-1. The stability and linearity of our system are ensured by temperature stabilization and holding the MZI in its quadrature point using a PID controlled piezo electric transducer (PZT) glued directly on a mirror in one arm of the MZI. Some of the relevant obtained results are reported in Fig. 1. Achieved limits of detection are in the low ppm region.