In the present study, an improved understanding of dynamic temperature and differential pressure phenomena during pressurized biomass gasification in a bubbling fluidized bed is used to develop a method for the early detection of bed defluidization. Experiments with silica sand as a bed material and three different biomass feedstocks, namely, birch chips, grot chips, and pine pellets, were conducted. For comparison, experiments using magnesite and dolomite as the bed materials were also conducted. The main variables potentially influencing bed fluidization, including K content and K/Si ratio in fuel feedstock, bed temperature, steam, and fluidization velocity, were considered in relation to their effect on bed defluidization time. Correlation of differential pressure and temperature at different positions in the bed with fluidized bed changes, such as char accumulation, sticky coating formation on particles, and char consumption, was evaluated and identified. The study demonstrates that under the operating conditions investigated, it is possible to determine the bed defluidization tendency fairly accurately using a combination of bed temperature deviation and the deviation of two series of dynamic differential pressure signals, measured at different bed positions.