Ecosystem soils influence the cycling of nutrients, movement and storage of water, and serve as an important global reservoir of carbon (C). The accumulation and storage of C in soils is a major factor in the global C cycle and is crucial for sustaining ecosystem health and function, yet gaps remain in our understanding of the processes that lead to the accumulation and stabilization of organic C compounds, i.e., soil organic matter (SOM), in soils. This information is essential for accurately projecting ecosystem health and condition with ecosystem process models such as GEM-TOPMODEL, and tracking the movement of nutrients and pollutants through soils, which influence both terrestrial and aquatic biota. Because vegetation, clay mineralogy, and environmental conditions play important roles in the production, stabilization, and sequestration of SOM, we developed a study to investigate their role in the accumulation of SOM across a range of forested soils in the Pacific Northwest. We selected 8 mature forest stands in the Oregon Coast Range Mountains and Cascade Mountains. These forests cover a range of forest types and environments. The eastern site is a Juniper (Juniperus occidentalis) forest growing on soils developed in ash. Also included are high- and low-elevation Douglas-fir (Pseudotsuga menziesii) forests growing on soils derived from old glacial deposits and recent glacially worked volcanics. Ponderosa pine (Pinus ponderosa) is represented by a stand growing in soil derived from volcanic and colluvial deposits. Coastal forests include Sitka spruce (Picea sitchensis) and Douglas-fir growing in soils from old marine sediments and basalts. Annual precipitation values range from less than 30 cm for the Juniper site to more than 300 cm for the coastal Douglas-fir and Sitka spruce sites. Soil samples were collected by horizon from the face of soil pits at each site. Soil particle size distribution and clay mineralogy was determined for each soil. We hypothesized that particle density is directly proportional to SOM stability, and separated SOM by density using sodium polytungstate. Total C and stable C isotope ratios in whole soil and in 4 density fractions were determined for each horizon. Accelerator mass spectrometry was used to measure the ¹⁴C in each horizon for the purpose of determining radiocarbon-based mean residence times of C, and infrared spectroscopy was used to characterize C chemistry. There was a 5-fold difference between the amount of C in the soil with the lowest soil C (Juniper forest) and the soil with the greatest soil C (Douglas-fir forest). Clay mineralogy of the sites is quite diverse, reflecting the soil parent material, age and weathering environment. The soils from the Coast Range Mountain sites are dominated by hydroxy-interlayered smectite, chlorite and paracrystalline clays. The soils from the Oregon Cascade Mountains have highly variable mineralogy both between the study sites and within the sampled soils. One site is dominated by smectites. Gibbsite is common in the old, high rainfall soils in the west Cascades. Soils derived from volcanic ash contain abundant paracrystalline material and hydrated halloysite in the lower solum. The amount of heavy-density fraction associated organic matter is related to the amount and kind of clay present in the soil. The mean ‘age' of soil C increased with depth in the soil. Soil C at depth was much older in the wet forest soils and the youngest C was found in the dry forest soils. However, the strongest relationship appears to be between ‘age' and the amount of clay, which is indicative of the protective and stabilizing nature of clay on SOM. These data along with environmental data and forest site history provide a unique way to evaluate the interacting factors that affect the accumulation and stabilization of SOM in forested soils in the Pacific Northwest USA.