Product and mechanistic studies were performed for the reaction of aromatic distonic peroxyl radical cations 4-PyrOO(center dot+) and 3-PyrOO(center dot+) with phenylacetylene (7) in the gas phase using mass spectrometric and computational techniques. PyrOO(center dot+) was generated through reaction of the respective distonic aryl radical cation Pyr(center dot+) with O-2 in the ion source of the mass spectrometer. For the reaction involving the more electrophilic 4-PyrOO(center dot+), a rate coefficient of k(1) = (2.2 +/- 0.6) x 10(-10) cm(3) molecule(-1) s(-1) was determined at 298 K, while a value of k(2) = (8.2 +/- 2.1) X 10(-11) cm(3) molecule(-1) s(-1) was obtained for the reaction involving the less electrophilic 3-PyrOO(center dot+). This highlights the role of polar effects in these reactions, which are likely of high relevance for processes in combustions and atmospheric transformations. The mechanism was studied by computational methods, which showed that radical addition occurs exclusively at the less substituted alkyne site to give the distonic vinyl radical cation 8. The latter undergoes a series of subsequent rearrangements/fragmentations that are similar for both isomeric PyrOO(center dot+). gamma-Fragmentation in 8 leads to the distonic aryloxyl radical cation PyrO(center dot+) and a singlet carbene 10. The product association complex [PyrO(center dot+) - 10] is the starting point for two important subsequent reactions, e.g., (i) rapid hydrogen transfer to form ketenyl radical 11 and the closed-shell species PyrOH(+), and (ii) oxygen transfer from PyrO(center dot+) to 10 that leads to a-keto aldehyde 13 and Pyr(center dot+), followed by hydrogen abstraction to give acyl radical 14 and PyrH(+). Additional major products are the dosed-shell aromatic carbonyl compounds 20 and 30 that result from multistep rearrangements in vinyl radical 8, which are terminated by homolytic bond scission and release of neutral acyl radicals.