Decay is severely affecting the collection of fossil marine reptiles in Britain's Natural History Museum. The fossils, dating from the Jurassic, are about 190 million years old, and range from isolated bones to complete skeletons, forming the largest and possibly the best such collection in the world. The specimens had been buried in an anaerobic environment favorable to their preservation but allowing the formation of microcrystalline iron pyrite. The high level of humidity present in the specimen cases and the aging museum caused the iron pyrite to oxidize producing sulfur compounds, including sulfuric acid which attacks containers and fossils. Steps taken to protect the fossils included the use of hygroscopic silica gel in the cases and the use of ammonia. The disadvantages of these methods are discussed. The museum's Palaeontology Laboratory has now developed a preservation technique using the chemical ethanolamine thioglycollate, which is 1) alkaline, 2) reactive with and a stabilizer for iron compounds, and 3) soluble in ethanol. The process of application and the satisfactory results are indicated. -- AATA