The BayCEER – Laboratory of Isotope Biogeochemistry is a keylab within BayCEER that provides with a wide range of analytical techniques the high-precision abundance analysis of stable isotopes of the elements H, C, N and O from solid, liquid and gaseous samples. The coupling of a thermal conversion (pyrolysis) device with an isotope ratio mass spectrometer (TC-IRMS) for the online analysis of H and O stable isotope abundance from solid and liquid samples is one of these techniques. Unfortunately, the application of H isotope abundance analysis is currently by far underrepresented in ecological and environmental studies. Reasons for this deficiency are (1) a lower precision of H isotope abundance analysis in comparison to other elements, (2) the requirement of sophisticated equipment, (3) high memory effects of H isotopes and (4) a bias by H equilibrium exchange processes. In my presentation I show how these limitations can be overcome. Two application examples from our own research demonstrate how H isotope abundance can be used as an elegant tool in investigations on food stuff origin and on the elucidation of partially mycoheterotrophic nutrition, i.e. the organic matter gain from a fungal source in addition to own photosynthesis. Partial mycoheterotrophy turns out as being much wider distributed than previously assumed among green-leaved and thus, putatively autotrophic plants.