Riboflavin phosphate (RFP) is an essential compound in the treatment of keratoconus - a degenerative, non-inflammatory disease of the cornea. Currently, the quantitative and efficient transport of riboflavin to the cornea is possible after mechanical removal of the epithelium. To avoid surgical intervention, it is therefore important to develop a method for quantitatively transporting riboflavin across the intact epithelium. In the present study, an RFP-loaded microemulsion was prepared, which could potentially function as an ocular drug delivery system crossing the eye epithelium. The specially designed water-dilutable microemulsion was based on a mixture of nonionic surfactants. Propylene glycol and glycerol acted as cosurfactant and cosolvent assisting in the solubilization of the RFP. The glycerol-rich water-free concentrate consisted of direct micelles for which glycerol served as the hydrophilic phase. In formulations with up to 40 wt% water, the hydrophilic surfactant headgroups and glycerol strongly bind water molecules (DSC and SD-NMR). Above 60 wt% water, globular, O/W nanodroplets, 14 nm in diameter, are formed (SAXS, cryo-TEM, and SD-NMR). The structure of microemulsions loaded with 0.14-4.25 wt% RFP (0.29-8.89 mmol per 100 g formulation) is not significantly influenced by the presence of the RFP. However, in the microemulsions containing 10-80 wt% water, the mobility of RFP in the microemulsion is constrained by strong interactions with the surfactants and cosurfactant, and therefore free transport of the molecule can be achieved only upon higher (>80 wt%) water dilutions. (C) 2015 Elsevier Inc. All rights reserved.