The hydrophobicity and surface polymer extracellular polymer characteristics of a range of foams and mixed liquor samples were examined and the species involved studied. The results show that, for samples dominated by Microthrix parvicella, the hydrophobicity of the solids was linearly related to the uronic acid content of the extracellular polymer polysaccharide. The stability of foams was increased significantly with the addition of polyvalent metal ions to the mixed liquors producing the foams.