Conversion in hot compressed water (e.g., 600 degreesC and 300 bar) is considered to be a promising technique to treat very wet biomass or waste streams. In this paper, a new experimental method is described that can be used to screen the operating window in a safe, cheap, and quick manner (one measurement takes about 5 min). Small sealed quartz capillaries (i.d. = 1 min) filled with biomass or model compounds in water are heated rapidly in a fluidized bed to the desired reaction temperature. The reaction pressure can be controlled accurately by the initial amount of solution in the capillary. After a certain contact time, the capillaries are lifted out of the fluidized bed, rapidly quenched, and destroyed to collect the produced gases for GC analysis. Results of measurements for formic acid and glucose solutions have shown that the technique is reliable enough for screening purposes including trend detection. For conversions above 30%, three identical measurements are sufficient to produce reasonably accurate average values with a confidence level of 95%.