The title reactions have been studied theoretically. At atmospheric conditions the acetaldehyde + HO2 reaction forms the alpha-hydroxyethylperoxy radical with rate constant of around 10(11) cm(3) mol(-1) s(-1). Further reaction of alpha-hydroxyethylperoxy with NO occurs for NO levels above around 10 ppb. Reaction with HO2 is therefore predicted to be a major sink of aldehydes in highly polluted areas, producing carboxylic acids and ozone. The reaction of vinyl alcohol with HO2 can be rapid at combustion temperatures, forming acetaldehyde + HO2 and alpha-hydroxyethyl + O-2 products. This reaction achieves the isomerization of vinyl alcohol to acetaldehyde at a faster rate than keto-enol tautomerization, given suitably high levels of HO2. (C) 2009 Elsevier B.V. All rights reserved.