We present a mid-IR laser-based photothermal spectroscopy (PTS) setup for the analysis of 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. The presented sensor system works reagent free and can be applied in an on-line format in the chemical industry as well as for fuel quality control, being industrial applications where traces of water need to be accurately determined, preferably in real-time. It thus holds great promise to replace the off-line Karl-Fischer titration method which is the current standard method for this application, but which entails important drawbacks such as being time consuming, requiring toxic reagents and producing waste. 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.