As part of the separation by froth-flotation of PVC and PET fr om waste bottles before recycling, the mechanism of action has been studied of a lignosulphonate used to selectively render the surface of PET hydrophilic. More specifically, an attempt has been made to determine the influence of cations. The results of the experimental study, based on flotation tests, wettability and zeta potential measurements, and XPS analyses, demonstrate the essential role of bivalent cations such as calcium in the hydrophilization action of lignosulphonate. Calcium may play, through electrostatic interactions, the role of a bridge between the surface of the plastics and the lignosulphonate, both negatively charged.