In the Mediterranean-type climates that occur over much of California, water is severely limited during the summer months. Interception of fog water can be a locally important source of plant-available water and groundwater recharge in foggy coastal areas, but the importance of fog precipitation in pedogenic mineral formation is unknown. Stable isotopes incorporated into pedogenic minerals may record water source and availability as they reflect equilibrium conditions with the soil solution at the time of formation. Soils in Pinus Muricata (Bishop Pine) canopy and adjacent grassland on Santa Cruz Island were compared on two parent materials, a chlorite schist and a rhyolitic tuff. Smectites dominate the fine clay (<0.2 μm) fraction in the soil formed on rhyolitic tuff, while kaolinite and halloysite dominate the fine clay fraction of the soils formed on schist. Stable ²H and ¹⁸O isotope analyses of precipitation (rain and fog) and soil solution samples indicate a depletion of the heavier isotopes in the rainwater compared to fog. Soil solution samples from surface horizons measured during precipitation events show a shift in isotope ratios to heavier values during fog events and lighter values during rain events, but generally increase with depth as packets of water influenced by evaporation infiltrate deeper into the profile. ²H values in fine clay phyllosilicates at both pine sites also reflect this increase.