A major exception to the general rule that isotopes have very similar chemical properties has been found. Hydroxymethylene (H-C=O-H) has been discovered by high-vacuum flash pyrolysis of glyoxylic acid (HCOCOOH) & isolation in a cryogenic matrix (Nature vol. 453 page 906). HCOH spontaneously rearranges to formaldehyde (H2CO) with a half-life of about 2 hours. However, the deuterated form HCOD is stable under the same conditions and has a calculated half-life of over 1000 years. The difference appears to be because HCOH rearranges by an unusual quantum mechanical tunnelling process.