The study presented is a follow-up work regarding the quartz capillary technique which was developed in our research group for performing screening tests of different biomass conversion routes. The aspects examined are flushing of the capillary with an inert gas, diffusion of gases through the quartz reactor wall and the catalytic activity of this wall. It was shown that hydrogen diffuses through the quartz reactor wall, which shows slight catalytic activity. This has, however, a negligible effect on previously obtained results for reactions at 250-800 degrees C up to 60 min reaction time. Flushing of the capillary microreactor with N-2 prior to sealing and reaction is essential, especially for low organics concentrations, to avoid partial product combustion by atmospheric O-2 in the capillary.