An extensive VOC data set was gathered as part of a photochemical oxidant field campaign conducted in the Phoenix air basin in the late spring of 1998. Sampling was done at the surface and by aircraft at midboundary layer height; in regions with emission sources and downwind in the urban plume. VOC concentration ratios were used to calculate photochemical age, defined as the time integrated exposure of an air mass to OH radical. Based on the VOC ratios of 15 compounds (with OH reactivity varying between acetylene and p, m-xylene), we present estimates for photochemical age and dilution factors for several regions within the air basin. Geographic trends are in agreement with the expectation that pollutants are transported in a generally eastward direction so that older and more dilute mixtures occur to the east of the city. Photochemical ages determined from aircraft samples agree with those determined at a downwind surface site. The bias in photochemical age that occurs because fresh pollutants are added to an aged mixture has been quantified by using a particle trajectory model. A combination of trajectory results (actual age of the pollutants in an air mass) and photochemical age yields an estimate of the average OH concentration experienced by the air parcel. OH obtained in this way is somewhat lower, but has the same trends as OH concentrations calculated using a photochemical box model that is constrained with observed concentrations coincident with the VOC samples.