It was demonstrated that a tandem micro-reactor coupled to a gas chromatograph-mass spectrometer (tandem micro-reactor-GC-MS) system can be successfully applied to study single-shot pyrolysis and on-line catalytic conversion (ex situ catalytic pyrolysis) of biomass samples using HZSM-5 catalyst. The pyrolysis of beech wood without using any catalyst produced various oxygenated compounds; prominently carbon dioxide, levoglucosan and acetic acid at all tested pyrolysis temperatures of 400, 500 and 600 degrees C along with aldehydes, ketones, and phenols. In the ex situ catalytic pyrolysis of the beech wood using HZSM-5, the yields of the oxygenated hydrocarbons were significantly reduced, whereas various aromatic compounds including furan, benzene and its alkylated derivatives, indane, indenes and naphthalenes were formed through catalytic conversion. The relative yields of these products changed depending on the pyrolysis temperature. The catalytic effect was observed even at the lowest catalyst loading of 2.5 mg. An increase in catalyst loading led to an increase in the relative yields of aromatic hydrocarbons. The catalyst de-activation test in the tandem micro-reactor showed that HZSM-5 catalyst can be repeatedly used with little loss of its activity. The tandem micro-reactor-GC-MS with HZSM-5 was also applied to other types of biomass (cellulose and microalgae). It was found that the relative yields of BTX (benzene, toluene, xylene) changed considerably depending on the type of biomass, probably due to the difference in the feedstock composition.