Extrafloral (EF) nectar is the foundation of many ant-plant interactions worldwide. EF nectar is a resource that encourages ant presence, in turn resulting in protection for the plant against herbivores. There is a paucity of research on this relationship and the plant structures that underpin it in Australian species. An examination of the morphology and anatomy of EF nectaries of an Australian cotton (Gossypium sturtianum) revealed a suberised layer in the nectaries which indicates solutes must travel through the symplasm. 14CO2 labelling showed the studied leaves were carbon sources and the EF nectar carbon cost is 1% of net photosynthates. Any environmental changes that affect EF nectar could have flow-on effects for ecological communities, for example rising atmospheric carbon dioxide (CO2) levels. This study tested the response of EF nectar to elevated CO2; the first investigation of elevated CO2 impacts on any EF nectar system. The total volume and composition of EF nectar did not change, but there was evidence of accelerated plant development and a change in EF nectar allocation within plants. Developmental changes due to elevated CO2 could affect the timing of EF nectar production which could have flow-on effects to ant mutualists and the defence of plants.