In fixed bed gasifiers, the char bed gasification zone where char is converted into syngas plays a major role in terms of efficiency and control of the process. This zone is particularly complex as many phenomena compete, i.e. heterogeneous and homogeneous chemical reactions, gas flow in porous medium and flow of solid particles. This paper investigates the mechanical and thermochemical behavior of the char bed gasification zone and focuses particularly on bed compaction. To achieve this, a low-density biomass char from wood chips and a high-density one from wood pellets were gasified in a pilot scale continuous fixed bed reactor. Measurements of profiles were taken along the char bed for temperature, gas species concentration, char composition, char bed density and char particle velocity using fine instrumentation and specific char and gas sampling techniques. In our operating conditions, the char bed reactive zone is 3 times longer for chips (45 cm) than for pellets (16 cm). We show that pelletization has no effect on: char bed compaction, final char conversion (about 95%) and syngas quality (16% H2 and 13% CO). Finally, we discuss char bed compaction and the main phenomena that control it in order to propose a line of inquiry for modeling.