Capillary electrophoresis (CE) is first applied to investigate the nature of interactions between water-soluble nonionic polymers and phosphate surfactants that have been mainly studied by conductometry and other means but got inconsistent results and conclusions. The interactions between polyvinylpyrrolidone (PVP) and monolauryl phosphate (MLP) are investigated by CE together with tensiometry, conductometry and titrimetry. The combined results make it clear that the interactions between PVP and MLP are pH-dependent and depend on different ionization states of MLP molecules. Therefore, MLP does complex with PVP under neutral and weakly acidic conditions rather than under alkaline condition, and the complexes are further recognized as the PVP-MLP pseudo-polyanions similar to other PVP-anionic surfactant systems. CE is confirmed as a new method for characterizing complexation other than a traditional separation technique. It is not only effective for explaining inconsistent results of interactions between nonionic polymers and phosphate surfactants, but also more sensitive than conductometry and tensiometry to investigate potential interactions between any polymers and any surfactants.