The main objective of this research is to separate a mixed post-consumer engineering plastic waste (PET, POM, and PVC) based on type of plastic using selective flotation separation. Depressing effect of wetting agent and aluminium sulfate on plastic is attributed mainly to the separation. POM is separated from the mixture waste when using 500 mg/l calcium lignosulfonate as a wetting agent at pH 7 and air flow rate of 220 ml/min. It renders POM more hydrophilic and causes higher wettability. This corresponds to its smaller contact angle. It is found from the results that POM has smaller contact angle than PET and PVC, respectively. Therefore, POM remains in the flotation solution while air bubbles are attaching to PET and PVC. To succeed in separation of PET from PVC by selective flotation, 500 mg/l calcium lignosulfonate and 0.01% (w/v) Al-2(SO4)(3) is added to a flotation solution at pH 5.8 and air flow rate of 180 ml/min. Negative charge ions on plastic surface are neutralized by aluminium ions. Additional, pH of solution is changed from 7 to 5.8 when 0.01% (w/v) adding Al-2(SO4)(3). Magnitude of zeta-potential of PET and PVC changes from negative to positive value when changing basic condition to acidic one. In this current work, point of zero charge of PET and PVC are investigated. At point of zero charge, surface of plastic has a complete hydrophobic property. pH of solution plays an important role in flotation separation. (c) 2006 Elsevier B.V. All rights reserved.