UNLABELLED: This paper presents a one-year record of in situ air-quality data from nine sites throughout Saudi Arabia. The data set is composed of hourly measurements of ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO) at six of the largest cities in Saudi Arabia (Riyadh, Jeddah, Makkah, Yanbu, Dammam, Hafouf) and two remote locations in the mountainous southwestern region of Alsodah for the year 2007. The authors found that international O3 and CO standards were routinely exceeded throughout the year at many sites, and that exceedances increased during Ramadan (Sep. 12-Oct. 13), the Islamic month of fasting when much of normal daily activity is shifted to nighttime hours. In general NO2 and CO levels were higher in Saudi cities compared to U.S. cities of comparable population, while O3 levels were lower. There was a general trend for O3 and NO2 to be negatively correlated in Saudi cities in contrast to U.S. cities where the correlation is positive, suggesting that ozone chemistry in Saudi Arabia is limited by volatile organic compound emissions. This may be caused by low biogenic emissions from vegetation. Pollutant levels were lower at most Saudi sites during the four day Hajj period (Dec. 18-21) but higher in Makkah which receives millions of visitors during Hajj. The authors also found that ozone levels were elevated during the weekend (Thursday & Friday) relative to weekday levels despite lower NO2, a phenomenon known as the "weekend effect." As little air quality data is available from Saudi Arabia in the English-language literature, this data set fills a knowledge gap and improves understanding of air quality in an important but under-reported region of the world. IMPLICATIONS: Air quality measurements at nine sites in Saudi Arabia provide a detailed look at spatial and temporal patterns of ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO). NO2 and CO levels increased in most cities during the fasting month of Ramadan, whereas O3 levels decreased. This led to an increased frequency of CO exceedances based on international standards. NO2 and CO also increased in Makkah during the Hajj pilgrimage. In general, NO2 and O3 levels were anticorrelated at most sites, suggesting that O3 chemistry is limited by volatile organic compound emissions throughout much of Saudi Arabia.