A water-in-supercritical CO2 microemulsion is a reverse micelle encapsulating a nanometer-size water droplet dispersed in supercritical CO2. In the microemulsion solution, water exists not only in the reverse micelle but also in the solvent CO2. For quantitative analysis of the water distribution, near-infrared spectra of water + CO2 and water + surfactant + CO2 mixtures were measured over a wide range of water/CO2 ratios from 0.1 to 1.0 wt % at 60 degrees C and 30.0 MPa. The stretching combination band of water was decomposed into two components, a sharp one peaked at 7194 cm(-1) assigned to monomeric water dissolved in CO2 and a broad one around 7000 cm(-1) corresponding to aggregated water in the microemulsion. Integrated molar absorptivities of these types of water were negligibly different from each other, despite the different hydrogen-bonding environments. The spectral decomposition revealed that water is distributed mainly into CO2 at water contents smaller than 0.5 wt % and then is introduced into the microemulsion after saturation of water in CO2 and full hydration of the surfactant headgroup.