In the work, the influence of the initial characteristics of feedstock on the yield, composition, and properties of fast pyrolysis products was studied. Samples of the original flax shive and its cellulose residue obtained as a result of alkaline hydrolysis were chosen as the raw materials for the investigation of fast pyrolysis to obtain useful products – phenyl propane derivatives (guaiacyl- and syringyl propanols, propenes, and propanes). The use of preliminary chemical processing of plant raw materials is aimed at maximizing the use of the initial potential of raw materials, as well as reducing energy costs and improving the quality of pyrolysis products. The paper discusses the dependence of the composition and properties of fast pyrolysis products on the process conditions – temperature, retention time of the raw material in the heating zone, and the diameter of the particle of the feedstock. The optimal conditions for fast pyrolysis for flax processing waste are temperature 700 °C, particle size 0.25 -1 mm, and particle residence time in the heating zone of 4.4 s. The results of a study of the process of fast pyrolysis of flax processing waste in the presence of a ZSM-5 catalyst are also presented. The presence of ZSM-5 zeolite increases the content of ethylene, hydrogen, and carbon dioxide, which is probably associated with the cracking of volatile products passing through the catalyst heated to high temperatures. The composition of liquid products of fast pyrolysis obtained using a thermocatalytic purification stage on the synthetic zeolite ZSM-5 was significantly reduced in the number of chemicals compared to the compositions of liquid products of fast pyrolysis obtained without a purification stage.