Properties of the surface-active substance polyoxyethylene(100) nonylphenol ether (NF100), in water and in deuterated water at room temperature, were investigated by static and dynamic surface tension measurements, using the fluorescence probe technique with pyrene and prodan and by NMR self-diffusion. It was found that at concentrations below 0.01% w/w, the substance could be described as a random-coil polymer at Θ conditions. The surface tension and fluorescence experiments indicated changes in surface adsorption and domain formations in the bulk at concentrations of between 0.01 and 0.06% w/w. At this higher concentration, no further adsorption took place at the air/water interface. However, the addition of more substance resulted in a bulk process of increased complexity involving aggregate formation and network formation, as was also revealed by the NMR self-diffusion experiments at even higher concentrations.