The secondary formation of HO2 radicals following OH + aromatic hydrocarbon reactions in synthetic air under normal pressure and temperature was investigated in the absence of NO after pulsed production of OH radicals. OH and HOx., (=OH + HO2) decay curves were recorded using laser-induced fluorescence after gas-expansion. The prompt HO2 yields (HO2 formed without preceding NO reactions) were determined by comparison to results obtained with CO as a reference compound. This approach was recently introduced and applied to the OH + benzene reaction and was extended here for a number of monocyclic aromatic hydrocarbons. The measured HO2 formation yields are as follows: toluene, 0.42 +/- 0.11; ethylbenzene, 0.53 +/- 0.10; o-xylene, 0.41 +/- 0.08; tri-xylene, 0.27 +/- 0.06; p-xylene, 0.40 +/- 0.09; 1,2,3-trimethylbenzene, 0.31 +/- 0.06; 1,2,4-trimethylbenzene, 0.37 +/- 0.09; 1,3,5-trimethylbenzene, 0.29 +/- 0.08; hexamethylbenzene, 0.32 +/- 0.08; phenol, 0.89 +/- 0.29; o-cresol, 0.87 +/- 0.29; 2,5-dimethylphenol, 0.72 +/- 0.12; 2,4,6-trimethylphenol, 0.45 +/- 0.13. For the allcylbenzenes HO2 is the proposed coproduct of phenols, epoxides, and possibly oxepins formed in secondary reactions with O-2. In most product studies only quantified coproducts were phenols whereas only a few studies reported yields of epoxides. Oxepins have not been observed so far. Together with the yields of phenols from other studies, the HO2 yields determined in this work set an upper limit to the combined yields of epoxides and oxepins that was found to be significant (0.3) for all investigated allcylbenzenes except m-xylene. For the hydroxybenzenes the currently proposed HO2 coproducts are dihydroxybenzenes. For phenol and o-cresol the determined HO2 yields are matching the previously reported dihydroxybenzene yields, indicating that these are the only HO2 forming reaction channels. For 2,5-dimethylphenol and 2,4,6-trimethylphenol no complementary product studies are available.