The effects of olefin carbon number distribution (chain length), bond position, and branching on sediment toxicity and anaerobic degradability were investigated. Specific carbon number olefins were tested individually for their sediment toxicity and then spiked in proportion to their molar distribution in a commercial C1518 internal olefin product containing predominately a distribution of C15 through C18. In other sets of experiments, toxicity contribution of C14 and lower (C10-C14) internal olefins to toxicity of the C1518 internal olefin product was investigated. Within the variability limits of the sediment toxicity test, specific carbon number olefins from C10-C14 showed similar toxicity. 10-day LC50s ranged from 28 to 56 mg/kg for C10 to C14 olefins when tested individually. C6 and C8 olefins did not exhibit toxicity at the concentrations tested presumably due to volatilization loss during sample preparation and test exposure. Concentrations of up to 4.8% C14 olefin (and lower) did not increase the toxicity of the commercial C1518 base fluid. C1518 base fluid showed no increased toxicity between the samples spiked with 7% to 25% C14. The effect of sediment organic carbon on olefins toxicity was examined in tests using formulated and natural field sediments. For each olefin, toxicity was highest in formulated sediment and lowest in the field sediment with the higher total organic carbon. In side-by-side tests examining the role of carbon to carbon bond position in sediment toxicity, LC50 values indicate no differences between α-olefin and internal olefin. With regards to anaerobic biodegradability, theoretical gas production appeared lower for a C16 α-olefin (60.3%) compared to C14 α-olefin (77.9%); however, only one data point was available for comparing the effect of chain length on anaerobic biodegradability. Anaerobic biodegradability was not affected by bond position for the one paired of isomers tested. Testing showed that increased branching of internal olefins adversely impacted both sediment toxicity and anaerobic biodegradation. The results presented in this study are part of on-going research and development activities aimed at understanding the relationships between chemical structure and environmental fate and effects of base oils for non-aqueous drilling fluids; which will provide the design basis for products with better environmental profile. Sustainable offshore drilling will require improvement in the environmental performance of these fluids.