An apparatus for the continuous treatment of an advancing web such as a cotton fabric with one or more reactive gases comprises one or more reaction chambers, each including fluid discharge means for directing a treating gas onto the web in the chamber and evacuation means disposed on both ends of the chamber to prevent both the fluid from escaping from the chamber and the fluid outside the chamber from entering the chamber. Heating means adapted to heat the continuously advancing web of fabric material may also be provided. Suitable heating means include a plurality of heated rotary drums. Such an apparatus may be used for the application of gases such as formaldehyde or sulfur dioxide or both to a cellulose fibercontaining fabric at an elevated temperature and in the presence of controlled amounts of moisture. In this manner, a previously applied N-methylol precondensate can be ''''wet fixed'''' in the fabric, thus making it possible to improve the wrinkle resistance and other shape-holding properties of the fabric while preserving good fabric strength after an eventual dry cure. Such a process can also be used directly to impart the desired shape-holding properties to the fabric, e.g., by crosslinking cotton by means of an acid-catalyzed formaldehyde reaction in the described apparatus.