The use of oxidation flow reactors (OFRs) has been rapidly increasing. We investigate organic peroxy radical (RO₂) chemistry in OFRs by kinetic modeling. It is found that, at low NO, UV intensity should be limited to avoid high radical levels leading to significant reaction of RO₂ with OH and negligible RO₂ isomerization, both of which are atmospherically irrelevant. We also develop two RO₂ fate estimators (for general use and for OFRs) to aid experiment design and interpretation.