In the framework of the European R&D program on non-nuclear energy (JOULE I) a research project started in 1990, aiming at in-situ analysis of product gas produced by coal gasifiers. Our contribution in the project deals with the development of solid electrolyte based electrochemical sensors. In this paper the design of the hydrogen measuring probe is presented together with the first emf-measurements in an Ar/H-2 atmosphere and in a simulated coal gasification atmosphere, while the latest results obtained in a real gasifier are discussed as well.