The formation of a peroxide accretion product (ROOR) has recently been shown to be a significant channel in self- and cross-reactions of peroxy radicals (RO₂) in the gas-phase. Here, we examine the formation of this accretion product in the self- and cross-reactions of RO₂ derived from the OH-initiated oxidation of propene, cis-2-butene, and methylpropene in the presence of ethene. We measure the formation rate coefficient of the various accretion products in each system relative to the formation rate coefficient of the ethene-derived ROOR, which was measured in our previous work. We find that the accretion product forms in all of the studied self- and cross-reactions. The measured ROOR formation rate coefficient for the self-reaction decreases by approximately an order of magnitude with increasing substitution, with average rate coefficients of 4.7 × 10⁻¹³ cm³ molec⁻¹ s⁻¹ for primary hydroxy peroxy radicals, 2.7 × 10⁻¹⁴ cm³ molec⁻¹ s⁻¹ for secondary hydroxy peroxy radicals, and 8.0 × 10⁻¹⁶ cm³ molec⁻¹ s⁻¹ for the tertiary hydroxy peroxy radical. The cross-reaction rate coefficients of secondary and tertiary peroxy radicals with primary peroxy radicals are both higher than the corresponsing self-reactions, and also decrease with increasing radical substitution. We estimate the branching fraction to the formation of the ROOR for these peroxy radical self- and cross-reactions and find that branching fractions range from 0.03–0.33, with self- and cross-reactions of primary peroxy radicals having the highest branching fractions. Finally, we compare the reaction and ROOR formation rate coefficients of self- and cross-reactions of small RO₂, and determine that the arithmetic mean of self-reaction rate coefficients provides a suitable method for estimating cross-reaction rate coefficients.

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