In this paper, the behavior of gas chromatography mass spectroscopy (GCMS) tar components in a three-stage water gas shift (WGS) unit is discussed. The GCMS tar measurements were carried out during the long-term operation (2250 h) of a WGS unit with tar-rich product gas from the commercial biomass steam gasification plant in Oberwart, Austria. In order to investigate the behavior of the GCMS tar components, four tar measurements were performed during the long-term operation of the WGS unit which employed a commercial Fe/Cr-based catalyst. The tar-rich product gas was extracted before reaching the scrubbing unit of the biomass steam gasification plant, therefore, the extracted gas contained a high amount of tar. In order to investigate the behavior of the GCMS tar in the WGS unit, the GCMS tar concentrations were determined at the inlet and the outlet of the WGS unit. The samples were taken during full load operation and during partial load operation of the WGS unit, respectively, the biomass steam gasification plant. In addition to the increase of the volumetric hydrogen content from about 40 % (d.b.) to 50 % (d.b.), the amount of GCMS tar was reduced (up to 38 %) as the gas passed through the WGS unit. No catalyst deactivation was observed. Furthermore, the efficiency of the hydrogen increase or the GCMS tar reduction did not depend on whether the operation of the WGS unit, respectively, the gasification plant was at partial load or full load.