The mechanism underlying gel layer formation on membrane surfaces from soluble and colloidal microbial products (SCMPs) produced under unfavorable operational conditions for membrane bioreactors (MBRs) has been investigated using supernatants from a bench-scale MBR. SCMPs can be grossly classified into gelling and nongelling SCMPs with the gelling fraction associated mostly with the polysaccharide content. The significant role played by multivalent metals in gel formation through metal-ligand complexation has been confirmed. Functional groups of the gelling SCMPs were determined by pH titration and zeta potential measurement as amine/phenolic sites (pK(a) 9.3 and 8.0), carboxylic sites (pK(a) 6.6, 4.9, and ca. 4.0), and phosphoric sites (pK(a) ca. 2.5). The carboxylic sites were more directly involved with multivalent cation complexation; however, the gelling propensity of the SCMP dispersion was minimally affected by pH change in the circum-neutral pH range, suggesting that the strong carboxylic sites were principally responsible for gel formation. The SCMPs demonstrated a high potential for gel formation given the high density of the strong carboxylic acid groups of about 0.44 mmol/g-TOC and a moderate calcium binding stability constant of about 4.9 x 10(3) M(-1).