This study for the UK Department for Transport reviews the potential sustainability impacts of low carbon fossil fuels, and develops a methodology for assessing their greenhouse gas (GHG) emissions. This GHG assessment needs to account for where the carbon would otherwise have been destined, had it not been used to make a new fuel product. Adopting this approach, this research illustrates that lifecycle carbon impacts of alternative fossil fuels range from significantly higher, to significantly lower emissions than conventional fossil petrol and diesel. The report also identifies a range of broader sustainability risks relating to air quality impacts, encouraging the production of more wastes, and of making an inefficient use of resources, for example, through contravening the waste hierarchy. If low carbon fossil fuels are given policy support, the study suggests that robust sustainability criteria should be in place to mitigate these risks.