As part of the Oxidizing Capacity of the Tropospheric Atmosphere (OCTA) project, an intensive measurement campaign was conducted in July/August 1993 at the high-altitude observatory Iza¿a, Tenerife. Measurements of NO, NO2, NOy, PAN, JNO2, CO, VOC, HCHO, H2O2, O3, and ROx were made to study the photochemical processes which control the oxidizing capacity of the remote troposphere. Special attention was paid to the processes controlling the budget of ozone. Diurnal changes in the concentration of the species resulted primarily from the transition between downslope flow (usually free tropospheric air) and upslope flow (a mixture of marine boundary layer air and free tropospheric air modified by island emissions). Median concentrations for downslope and upslope conditions were NOx (47/76 parts per trillion by volume (pptv)), NOy (392/519 pptv), peroxyacetylnitrate (PAN) (10/23 pptv), CO (89/92 parts per billion by volume (ppbv), ethane (499/486 pptv), propane (35/40 pptv), ethene (25/31 pptv), isoprene (0/60 pptv), HCHO (1.1/1.4 ppbv), H2O2 (2.4/2.1 ppbv), and O3 (40/38 ppbv). Maximum amounts of ROx were measured around noon and reached values up to 70 pptv with no observable signal in the night during downslope conditions. ¿ 1998 American Geophysical Union