The goal of this study is to evaluate the role of biomass heating time on bio-oil yield during fast pyrolysis in a free fall reactor. Biomass heating times were controlled using several configurations for conveying solids from the bottom of the free fall reactor. In all tests reactor wall temperature was 550 degrees C, sweep gas flowrate was 18 L/min, and heated reactor length was 2.77 m, which set the minimum biomass heating time of approximately 1.4 s (particles falling downward in sweep gas). Analysis of products included solid, liquid and gas mass yields; proximate analysis of the solids; and water-soluble sugar and carboxylic acid content of the liquid. These experiments showed that biomass continued to pyrolyze well beyond the few seconds frequently assumed to be adequate to achieve fast pyrolysis. Bio-oil yield increased with biomass heating times, even for times exceeding 40 s. Analysis of liquid products suggests that hemicellulose, cellulose, and lignin all contributed toward the longer than expected pyrolysis times. (C) 2015 Elsevier Ltd. All rights reserved.