Newly developed and future instruments display more and more sensitivity to contamination. Although current monitoring methods allow the punctual and ex-situ estimation of the deposited contamination levels on sensitive surfaces, it does not exist dedicated means to measure in situ and real-time the contamination deposition. Besides, conventional monitoring is performed usually until encapsulation, in the best case under fairing prior launch, and from launch to end of-life, only estimation by modelling is performed. Overall, there is currently a lack of representativeness and only a partial coverage of the contamination monitoring over the whole mission, from manufacturing to in-orbit operation. Having the capability to perform in-situ and real-time monitoring of the contamination will allow the determination of the actual levels deposited on the sensitive surfaces to anticipate the triggering of mitigation actions to ensure optimal system performances until end-of-life. These mitigation actions could be implemented both on-ground (cleanroom class improvement, additional protection installation, purging, ...) and in-flight (optimisation of the commissioning and decontamination phases, ...). In this article, we present a project in collaboration with European Space Agency, aiming at developing a sensor breadboard at TRL 3 able to quantify the deposited molecular contamination (hydrocarbon, ester, and, Silicones) to provide reliable in-situ and real-time data during assembly integration and testing with the possibility to be further developed for in-flight. The sensor is based on a piezo-MEMS Fabry-Perot Interferometer (FPI) developed and commercialized by Tunable AS for gas detection. The measurement system has been developed, mounted and tested on reference samples and its performances evaluated.