The sodium salt of the di-chained anionic surfactant bis-2-ethylhexylsulfosuccinate [Aerosol-OT or Na(AOT)] stabilises essentially monodisperse, spherical water-in-oil microemulsion (w/o) droplets in alkanes over a wide range of pressure, temperature and composition. In order to investigate the effect of change in counterion charge and size on the microemulsion properties, we have replaced the Na⁺ counterion by doubly charged ions. The surfactant is then M²⁺(AOT)₂·nH₂O: M is from the series Mg²⁺, Ca²⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺, and n is the number of water ligands associated with the surfactant molecule. The value of n was determined by FTIR and depends on M²⁺, but can be between 2 and 8 per molecule of M²⁺(AOT)₂ depending on the nature of M. The ion replacement, assessed by UV–VIS spectrophotometry, is ca. 100% efficient. The effect of temperature on the phase stability of the single-phase M²⁺(AOT)₂ water-in-oil (w/o) microemulsion systems is negligible, in contrast to that observed for the corresponding Na(AOT) system. The structure and properties of the microemulsion are found to be dependent on the counterion identity. Small-angle neutron scattering (SANS) and viscosity measurements provide evidence for the existence of rod-shaped aggregates for Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ at low water constants given by w=[H₂O]/[AOT]≈ 5, whilst for Mg²⁺ and Ca²⁺ spherical aggregates are present as for Na⁺. On further addition of water at constant surfactant concentration (w > 10) with Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺ the aggregates undergo a shape change, and a more spherical structure is favoured. The results may be explained in terms of the interaction of the different counterions with the SO^–₃ head group of the surfactant.