The mechanisms for the reaction of OH with the unsaturated aldehydes, acrolein and crotonaldehyde, have been determined at I atm total pressure in the presence of NO, using an environmental chamber/FTIR spectrometer system. Products observed in the OH-initiated oxidation of acrolein were CO, CO2, CH2O, HOCH2CHO (glycolaldehyde), and HCOOH, while the major products identified in the OH-initiated oxidation of crotonaldehyde were CO, CO2, CH3CHO, and HC(O)CHO (glyoxal). Also observed were two PAN-type species, identified as CH2=CH-C(O)O2NO2 (APAN) from acrolein oxidation and CH3-CH=CH-C(O)O2NO2 (CPAN) from crotonaldehyde. The near-complete mass balance obtained in these experiments allows for a quantitative assessment of the branching ratios for abstraction and addition in these reactions. It is shown that about 68% (50%) of the OH reaction with acrolein (crotonaldehyde) proceeds via abstraction of the aldehydic H, with the remainder occurring via addition to the double bond. The data allow for a more accurate assessment of the atmospheric source strength of APAN, a species which has now been identified in ambient air. Trends in the reactivity of acrolein and its methylated derivatives, methacrolein and crotonaldehyde, are also discussed; data are shown to be consistent with structure-reactivity considerations.