The reaction of propene with mercuric salts of some strong oxy-acids has been shown to give acraldehyde as a major product. This reaction proceeds through a 1:1 adduct which in aqueous solution is probably of the form CH3·CH(OH)·CH2·Hg+X–. The rate of reaction of this species with mercuric ions has been measured. The olefins but-1-ene, isobutene, pent-1-ene, and 2-methylbut-1-ene undergo an analogous reaction, more rapidly.