Thymol has proved the most suitable fungicide for bulk treatment of documents in the Somerset Record Office for 23 years. The treatment is carried out in a specially constructed chamber -- a room-with the documents loaded on trolleys. The vapor is generated by six 100-watt light bulbs under corresponding trays of thymol. The normal cycle consists of vapor being generated for two two-hour periods during the 48 hours the documents are in the chamber. In extreme cases, longer treatment is given. The chamber is provided with an extraction system, which is used before workers enter it. No return of mold has been detected on the documents in the strongrooms.