Proton NMR measurements were conducted on a U-type model quaternary nonionic microemulsion containing varying amounts of water and having a fixed weight ratio of surfactant [octaethylene glycol mono-n-dodecylether; C-12(EO)(8)], cosurfactant (pentanol), and oil (dodecane). Chemical shift and T-1 relaxation time data were used to obtain information about the microemulsion properties and structure. A slow exchange was observed between the hydroxyls of water, C-12(EO)(8), and pentanol in microemulsions containing up to 10 wt% water, This is precedence of the hydroxyl-water interaction over hydration of the ethylene oxide (EO) groups of the surfactant. The fraction of bound water was calculated from the chemical shift and T-1 data. For the model system, chemical shift measurements indicated an inversion from W/O to O/W microemulsion at about 55-60 wt% water, in agreement with the microviscosity values evaluated from T-1 data : 1.1 cP (55 wt% water) compared to 4.7 cP (30 wt% water) and 12.7 cP (10.7 wt% water). The irregularities in the monotonic dependence of the chemical shifts on water content in the microemulsion at 18 and 55 wt% water were interpreted to possibly reflect structural changes that took place in solution.