Source apportionment modeling provides valuable information on the contributions of different source sectors and/or source regions to ozone (O<sub>3</sub>) or fine particulate matter (PM<sub>2.5</sub>) concentrations. This information can be useful in designing air quality management strategies and in understanding the potential benefits of reducing emissions from a particular source category. The Comprehensive Air quality Model with Extensions (CAMx) offers unique source attribution tools, called the Ozone and Particulate Source Apportionment Technology (OSAT/PSAT), which track source contributions. We present results from a CAMx source attribution modeling study for a summer month and a winter month using a recently evaluated European CAMx modeling database developed for Phase 3 of the Air Quality Model Evaluation International Initiative (AQMEII). The contributions of several source sectors (including boundary conditions representing transport of emissions from outside the modeling domain) to O<sub>3</sub> or PM<sub>2.5</sub> concentrations in Europe were calculated using OSAT and PSAT, respectively. Evaluation focused on 16 major cities and on identifying source sectors that contributed above 5 %. Boundary conditions have a large impact on summer and winter ozone in Europe and on summer PM<sub>2.5</sub>, but are only a minor contributor to winter PM<sub>2.5</sub>. Biogenic emissions are important for summer ozone and PM<sub>2.5</sub>. The important anthropogenic sectors for summer ozone are transportation (both on-road and non-road), energy production and conversion, and the industry sector. In two of the 16 cities, solvent and product also contributed above 5 % to summertime ozone. For summertime PM<sub>2.5</sub>, the important anthropogenic source sectors are the energy sector, transportation, industry, and agriculture. Residential wood combustion is an important anthropogenic sector in winter for PM<sub>2.5</sub> over most of Europe, with larger contributions in central and eastern Europe and the Nordic cities. Other anthropogenic sectors with large contributions to wintertime PM<sub>2.5</sub> include energy, transportation, and agriculture.