A static fluorescence quenching technique has been used to determine the discrete droplet concentration, and by implication droplet size, in oil-continuous microemulsions stabilised by Aerosol OT (AOT). In particular (i) the nature and structure of water-in-oil microemulsions in the critical region have been investigated for the H2O–AOT–dodecane system and (ii) the effect of additives (benzyl alcohol and toluene) on the size of droplets in the single-phase system has been studied. From these studies it is demonstrated that the static fluorescence quenching technique is particularly appropriate for the investigation of changes in size domains. The results for the critical system suggest that there is no significant increase in droplet size in the single-phase system as the critical point is approached. However, evidence is obtained for facilitated solute exchange between droplets in the critical region. Small systematic increases in droplet concentration (decreases in droplet size) are readily detected on addition of benzyl alcohol and toluene to a single-phase system at compositions far removed from the critical (phase-transition) region. The static fluorescence quenching method with Ru(bp)2+3 as fluorescer and Fe(CN)3–6 as quencher is shown to be optimal for the determination of size domains in the 1–2 nm region. The quenching rate constant within a droplet is slower than in bulk aqueous solution at high ionic strength. At high concentrations of benzyl alcohol the fluorescer partitions to the interfacial region which allows facilitated exchange of Ru(bp)2+3 between droplets.