Abstract: A process for treating workpieces in a normally closed treatment chamber. The process comprises providing a pressure within the chamber which is less than atmospheric and providing an unsaturated treatment vapor within the chamber. A workpiece is then moved into the chamber to be treated by the unsaturated vapor. The process is particularly suited for the vapor phase soldering of printed circuit boards. The unsaturated vapor in the chamber is heated to a temperature higher than the melting point of the solder used and the circuit boards with solder thereon are passed through the vapor in the chamber to reflow the solder.The invention is also directed toward an apparatus for carrying out the process.