This spring, California refinery crack spreads for gasoline and diesel dipped below average despite shrinking refinery capacity on the West Coast (PADD 5). Crack spreads are the difference between refined product prices and an equivalent volume of crude oil. We use them as a measure to estimate refinery margins based on commodity market conditions. The decline in West Coast crack spreads stems from growing regional gasoline inventories and the increasing use of biofuels in place of conventional, petroleum-based diesel fuels in California.