Abstract: An autoclave is provided for heat treatment of a load whose position relative to the autoclave, whose cross-section and/or whose thermal characteristics may vary along the load, e.g. large panels for an airliner. The autoclave comprises a chamber for receiving the load, a wall of said chamber providing one end thereof and a door providing the other end of the chamber and giving access for insertion and removal of the load. Means is provided for heating gas in the chamber, and a plurality of gas circulation means are provided spaced along the length of the autoclave and each producing a zone for circulation of heating gas. Means is provided for independent control of the rate of heat transfer between the heating gas and the load in said zones and said gas circulation means is arranged to produce a pattern of circulation in which heating gas impinges non-axially onto the load.