Pyrolysis of acid-leached bagasse (515 °C) and pinewood (485 °C) has been carried out in the pressure range from 5 × 10-3 to 100 kPa in a screen-heater, designed for nearly isothermal operation and rapid quenching of reaction products. At the lowest pressure, i.e., by maximizing the escape rate of products away from the hot reaction zone, 73% of the poly-C6-sugars in bagasse were recovered in the liquid product as C6-anhydrosugars (C6aS) with degree of polymerization between 1 and 6 (DP1 to DP6). A mathematical model, including reactions and mass transfer, was able to predict the measured decrease in the total yield of C6aS and the shift to lighter C6aS in the DP-distribution as a function of increasing pressure. The effect of the hot vapor residence time on the DP-distribution of the C6aS was investigated by pyrolyzing pinewood in a fluidized bed. At identical pressure (50 kPa) and temperature (485 °C) the total yield of C6aS was the same for the screen-heater and fluidized bed while the DP-distribution shifted to DP1 as a result of the higher hot vapor residence time in the fluidized bed, which could be described by assuming first order kinetics for all possible cracking reactions of C6aS in the vapor phase.