Aquatic microalgae have high potential for production of bio-chemicals, liquid transport fuels and charcoal. Their main advantage over existing energy crops is that they have faster growth rates and do not compete with food production. In this study six species of microalgae (Tetraselmis chui, Chlorella like, Chlorella vulgaris, Chaetocerous muelleri, Dunaliella tertiolecta and Synechococcus) were selected, presenting a broad cross-section of physical characteristics and known behaviour under cultivation. The objective of this work was to ascertain differences in thermal conversion behaviour between the microalgae species under slow pyrolysis conditions. The samples were first analysed with a Computer Aided Thermal Analysis (CATA) technique at a standard heating rate of 10 °C/min. For all species, the energy required to achieve thermal conversion was found to be approximately 1 MJ/kg. Gas chromatography was then applied to measure the evolution of biogas compounds with temperature. The heat of combustion of the biogas compounds was estimated to vary significantly between species, ranging from 1.2 to 4.8 MJ/kg. Pyrolysis oil product yields were also estimated at 500 °C. The oils produced at this temperature were collected and their molecular weight distribution assessed by Matrix Assisted Laser Desorption/Ionisation (MALDI). The species were found to produce up to 43% by volume of bio-oils. In all samples the char fraction remained above one third of total sample weight.