A new water-in-oil microemulsion, consisting of the non-ionic surfactant Triton X-114, cyclohexane as the oil phase and 1.0 M AlCl₃ aqueous solution as the aqueous phase, was developed and used to synthesise aluminium oxide (Al₂O₃) nanoparticles. The partial phase diagram of this ternary system was also determined at 38 °C. Syntheses of Al₂O₃ nanoparticles were carried out in a range of microemulsions with a fixed oil∶surfactant ratio of 70∶30 (w/w), but with different aqueous phase contents. For the purposes of comparison, Al₂O₃ particles were also synthesised in the corresponding emulsion and by direct precipitation from aqueous solution. Nano-sized Al₂O₃ particles were only obtained via microemulsions, whereas the emulsion route led to hollow ball-shaped particles with sub-micron apparent sizes and the direct precipitation process resulted in angular particles about 250 nm in dimension. The nanoparticles synthesised in the microemulsion with 20 wt% aqueous phase content have smaller particle sizes (5–15 nm) and a lower transformation temperature into α-form Al₂O₃ crystallites. Pure α-Al₂O₃ nanocrystals were obtained by calcination at 1000 °C for 12 h, while the nano-sized dimension of the particles retained though the apparent particle size increased to some extent. This transformation temperature is about 200 °C lower than that for the Al₂O₃ particles synthesised by direct precipitation.