Prompt gamma activation analysis (PGAA) has been further developed to analyze reacting components inside a chemical reactor. The new method, in situ PGAA, was used to determine the hydrogen-to-palladium molar ratio under various conditions of palladium-catalyzed alkyne hydrogenation. The H/Pd molar ratio was successfully measured in the range of 0.1-1.0 in an approximately 2-g catalytic reactor containing a few milligrams of palladium catalyst. The amount of hydrogen was only a few tens of micrograms, and the detection limit was approximately 5 microg, i.e., at ppm level compared to the whole reactor. The description of the device, methodological developments, a feasibility study, and results of a series of catalytic measurements are presented.