With the purpose of demonstrating a process for pressurized entrained flow gasification for pulverized biomass, the aim with this work was to characterize the conditions inside the gasifier. To gain a broader understanding, it was important to extract both gases and particulate matter from the hot reaction zone. The objectives were, therefore, to (1) develop a sampling system capable of extracting both gas and particulates from the gasifier, (2) study the production of particulate matter as well as its composition and size distribution as a function of different operating conditions, and (3) extract time-resolved data for the syngas species (CO, CO2, and CH4) in order to study the compositional variance. The results indicated that the syngas heating value was lower at the sampling position in the gasifier compared to the heating value measured downstream of the quench cooler. The difference was most probably an effect of ongoing gasification of carboneous solids downstream of the sampling position in the gasifier. Furthermore, it was concluded that the fuel feedrate was fluctuating, most likely because of heterogeneity in the fuel powder and/or the challenges in the fuel feeding system itself. With regards to particulate matter in the syngas, it was shown to mostly consist of soot. The soot yield was significantly reduced by increasing γ. The reactor core sampling system proved superior to the traditional sampling system downstream of the quench with regard to measuring soot yield at different operating conditions of the gasifier. Finally, it was concluded that the submicron fly ash particles from oxygen blown biomass gasification contain high propotions of refractory elements (e.g., Ca, Mg, and Si) in addition to the more volatile elements (e.g., K, Na, S, and Cl). This is probably due to extremely high temperature in the flame and substoichiometric condition in the gasifier, which may promote vaporization of refractory elements during char gasification.