This thesis focuses on measurements of peroxy radicals, which are reactive intermediates in the oxidation of organic species in the troposphere. Laboratory experiments with a Peroxy Radical Chemical Amplification (PERCA) instrument have shown that the claim length (CLN) of the inlet amplification reaction decreases markedly with increasing specific humidity. The magnitude of the effect is reduced by warming the inlet to above ambient temperatures. Modelling of the PERCA inlet chemistry using the measured CLN(wet)/CLN(dry) ratios for given temperature and humidity gave fitted hydroperoxy radical (HO2) wall-loss rate coefficients in good agreement with literature measurements, which would suggest that enhanced wall loss of HO2 in humid air are sufficient to explain the observed CLN-humidity effect. A study of night-time chemistry at Mace Head in Ireland during the Eastern Atlantic Spring Experiment, 1997 (EASE 97) has shown that the oxidation of alkenes at night by reaction with ozone was greater by a factor of four than that by reaction with the nitrate radical, NO3, over the whole EASE 97 data set. The sustained mixing ratios of peroxy radicals observed at night demonstrated a requirement for substantial radical production processes, in the absence of daytime photochemistry. Peroxy radial rate of production analyses showed that ozone reactions produced more peroxy radials over the whole night-time period (defined as where [NO3] 0) than nitrate reactions (66 versus 34%, on average). However, the two mechanisms were found to operate at a similar rate in the middle of the night, when [NO3] was highest.