The OH-initiated oxidation of methacrolein, a major product of isoprene oxidation, has been studied in an environmental chamber using FT-IR spectroscopy. Products observed (which account for more than 90% of the reacted carbon) were CO, CO2, hydroxyacetone, formaldehyde, and methacryloylperoxynitrate (MPAN). It is determined that the attack of OH on methacrolein occurs 55% of the time via addition to the double bond, and 45% via abstraction of the aldehydic hydrogen atom, in agreement with a previous study. The end products of the abstraction channel are identified and quantified for the first time, and the mechanism of their production discussed. ¿ 1999 American Geophysical Union