Diurnal variations in concentrations of nonmethane hydrocarbons (NMHCs) were measured during June and early July of 2001 near the top of the Bank One Building (elevation 139 m) in Phoenix, Arizona. Ambient air was collected in SummaŽ passivated stainless steel canisters over five-minute intervals, every half hour between 0500 and 0930 local time (LT) and every three hours between 1000 and 0400 LT. Samples were also taken from the 4th floor of a nearby hotel during the morning rush hour. The samples were analyzed by a cryogenic preconcentration/high-resolution gas chromatographic technique with flame ionization detection. Distributions of NMHCs near the top of the Bank One Building will be compared with those measured at the surface to examine photochemical processing of the NMHCs.