The application of the miniemulsion process to synthesize polyurethane dispersions with diameters of about 200 nm in a one-step procedure, free of molecular weight restrictions, is described. With IR and NMR spectroscopy, it is shown that the reaction of the diisocyanate with the diol is the main reaction, whereas the reaction with water is only a side reaction. Differential scanning calorimetry measurements and transmission electron microscopy are used to characterize both polymer and latex properties.