Abstract: The invention relates to a process for acetylation of lignocellulosic fibres (LF) using an acetylating agent comprising acetic anhydride at a temperature of above 140.degree. C. and a pressure of 100-150 kPa wherein raw or substantially raw LFs are treated with a superheated acetylating agent comprising at least 20 % w/w acetic anhydride for a duration of at least 1.5 minutes in an acetylation reactor (3), the treatment with superheated acetylating agent also ensuring that the acetylated LFs are substantially free from occluded, adsorbed or absorbed acetylating agent, the amounts of which are less than 5 % w/w of the acetylated LFs recovered from the base of a circulation cyclone (5), so as to substantially acetylate the LFs to achieve a weight gain of at least 2 %. The LFs may optionally be pre-treated, prior to being treated with the superheated acetylating agent, preferably by spraying, for a very short period of time.

Abstract: The invention relates to a process for the acetylation of lignocellulosic materials (LM) by (a) bringing the LM into intimate contact with an acetylating agent comprising acetic anhydride as the major component at a temperature from 80.degree.-140.degree. C. and (b) bringing the acetylated LM from step (a) into contact with a heated gas inert under the reaction conditions in a stripper at a temperature above 140.degree. C. and reducing the acetic acid or acetic anhydride content of the acetylated LM produced in step (a) to below 10% by weight by stripping. Products fabricated from LMs so treated are highly dimensionally stable.