We present an indirect mid-IR sensing scheme based on a 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. Our 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. [1] The PTS signal is directly proportional to the laser emission intensity of a modulated mid-IR excitation source and to the sample absorption: absorption leads to a periodical heating and cooling which leads to expansion and a sample refractive index change (Δn).[2] We use a second laser source to probe the consequent refractive index change (Δn). The challenge lies in detecting the smallest Δn. Our approach consists in an interferometric configuration. In particular, our liquid PTS IR sensor exploits 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. When benchmarking the system against commercial FTIR spectrometers it is shown to be in excellent agreement with regards to band shapes, positions and relative intensities and to compare favourably in terms of sensitivity. Achieved limits of detection are in the low ppm region.