Nonylphenol ethoxylates (NPEs) are non-ionic surfactants used in detergents and as emulsifiers. NPEs are endocrine disruptors with estrogenic effects, creating severe issues with aquatic organisms. The risk of bioaccumulation is very high, and its degradation products are very toxic. Its use is therefore strictly regulated. This Application Note describes the determination of nonylphenol ethoxylate by potentiometric titration with sodium tetraphenylborate using the NIO surfactant electrode.