The behaviour of p-nitrophenol (PNP) absorbed at the dodecane/water interface and the interaction with tri-n-butyl phosphate (TBP) were studied by surface second harmonic generation (SHG). The polarisation dependence of the PNP SHG signal, which is the same for all concentrations of PNP and TBP, is best described in terms of a limiting weak orientational order. In the presence of TBP, the SHG signal initially increases with PNP concentration, passes through a maximum and then it decreases. At low PNP concentrations, increasing the TBP concentration leads to an increase in the SHG intensity, but for higher PNP concentrations the intensity decreases. The free energies of adsorption for PNP and TBP to the dodecane/water interface were determined using Frumkin and Langmuir isotherms. The kinetics of the interaction between TBP and PNP was observed by monitoring the SHG signal along a liquid–liquid flow cell and found to be first-order at high interfacial coverage with rate constant, k, of 0.1±0.01 s⁻¹.