Selective dissolved-air flotation for the separation of medium-chain-length polyhydroxyalkanoate (PHA) inclusion bodies (IBs) from Pseudomonas putida cell debris is investigated. Measurements show that both P. putida cell debris and PHA IBs have an iso-electric point of approximately pH 3.5. Selective aggregation and as a result selective flotation of PHA IBs was observed near this pH. Qualitative prediction of the aggregation behaviour was possible on the basis of the Van der Waals, hydrophobic and electrostatic interactions. In some cases however, the stability of the suspension could not be explained with these forces alone. It was therefore suggested that additional interactions. Such as steric/brush effects. play an important role in the aggregation process. (c) 2005 Elsevier Inc. All rights reserved.