The Triassic-Jurassic Boundary marks one of the largest mass extinction events of the Phanerozoic. Across the boundary, a rise in carbon-dioxide levels and global temperatures are hypothesized to have driven significant environmental changes inducing a major floral turnover, causing vegetation structure, composition and leaf morphology to alter, and inferred wildfire activity to increase. An example of these changes can be observed at the Astartekløft site in East Greenland, where previous work identified a change in flora from broad-leaved conifer dominated to an assemblage dominated by narrow leaved conifers, coeval with a five-fold increase in charcoal abundances. Variations in carbon-dioxide concentrations have been shown to be capable of influencing leaf chemistry. It could therefore be hypothesized that carbon-dioxide-driven climate changes across the Triassic-Jurassic boundary may have been capable of not only inducing changes in leaf morphological fuel properties, but also variations in biochemical properties that are both capable of altering wildfire behaviour. In order to assess this, we selected three plant species that have ancient evolutionary origins and correspond to the dominant leaf morphotypes of litter-forming vegetation observed at the Astartekløft site across the Triassic-Jurassic boundary. We grew these species in current ambient and high carbon-dioxide (Triassic-Jurassic boundary) atmospheric conditions and analysed variations in the chemistry of the leaves, using gas chromatography mass spectrometry, and assessed aspects of their flammability using micro-calorimetry. These data were used to inform a fire behaviour model to produce estimates of variations in fire behaviour, such as surface fire spread, flame length and fireline intensity across the Triassic-Jurassic boundary at Astartekløft. Our results reveal a change in leaf chemistry that is expressed as a suppression of volatile content in the three species grown under elevated carbon-dioxide concentrations, compared to those grown under ambient conditions. By accounting for these variations in a fire behaviour model, we estimate that fire behaviour was more extreme prior to the increase in carbon-dioxide across the boundary, suggesting a switch from a period of infrequent but intense fast-moving surface fires during the Triassic, to a period of frequent but low intensity and slow spreading fires during the earliest Jurassic. Our results indicate that that increases in carbon-dioxide concentrations may have impacted leaf chemistry and thus flammability, and may therefore have played an interesting role in determining fire behaviour characteristics during this marked period of Earth’s past.