During the Texas Air Quality Study II (TexAQS 2006) campaign, a PEroxy Radical Chemical Amplifier (PERCA) was deployed on the NOAA research vessel R/V Brown to measure total peroxy radicals (HO[subscript 2]+Σ RO[subscript 2]). Day-time mixing ratios of HO[subscript 2]+Σ RO[subscript 2] between 25 and 110 ppt were observed throughout the study area – the Houston/Galveston region and the Gulf coast of the US – and analyzed in relation to measurements of nitrogen oxides, volatile organic compounds (VOC) and photolysis rates to assess radical sources and sinks in the region. The measurements of HO[subscript 2]+Σ RO[subscript 2] were used to calculate the in-situ net photochemical formation of ozone. Measured median values ranged from 0.6 ppb/h in clean oceanic air masses up to several tens of ppb/h in the most polluted industrial areas. The results are consistent with previous studies and generally agree with observations made during the previous TexAQS 2000 field campaign. The net photochemical ozone formation rates determined at Barbours Cut, a site immediately south of the Houston Ship Channel, were analyzed in relation to local wind direction and VOC reactivity to understand the relationship between ozone formation and local VOC emissions. The measurements of HO[subscript 2]+Σ RO[subscript 2] made during the R/V Brown TexAQS 2006 cruise indicate that ozone formation is NO[subscript x]-limited in the Houston/Galveston region and influenced by highly reactive hydrocarbons, especially alkenes from urban and industrial sources and their photo-oxidation products, such as formaldehyde. ; Peer-reviewed ; Publisher Version ; 44341