Composite materials are increasingly being used in applications in which their fire response is a critical consideration. Environmental concerns motivate the use of natural fibres, but they represent an additional fuel source for combustion, and their composites will perform worse in flammability testing compared to synthetic alternatives. An experimental study is presented, comparing flax fibre reinforced epoxy samples to composites formed from glass fibre reinforcements of similar architecture, reinforcing the same resin system. The flax composites ignite earlier, release greater levels of heat, and their structure deforms significantly during combustion. Further studies are required to increase understanding of the combustion process, if efficient mechanisms are to be found to reduce flammability of natural fibre composites to acceptable levels.