An easy and cost effective method of monitoring the fermentation in biogas reactors is possible with the Biogas-Reactor Kit. The kit contains a calibrated syringe [sample volume 15 ml] and a package of ten detector tubes for measuring carbon dioxide in the range of 5 Vol.-% up to 40 Vol.-%. The main components of biogas CO2 and methane will be calculated as difference from measured CO2 value and 100 Vol.-%. Other components of interest, e.g. hydrogene sulfide in trace amounts, can be measured with standard type detector tubes.