This work describes a method for on-line monitoring of biomass production, acetate consumption and intracellular polyhydroxybutyrate (PHB) storage by mixed microbial Cultures (MMC). The method is based oil reliable and easily available on-line measurements, namely pH, dissolved oxygen, dissolved carbon dioxide, on-line respirometry and on-line titrimetric analysis. Biomass production refers to active biomass growth and also to the synthesis of extracellular polymeric substances (EPS). The composition and kinetics of EPS synthesis has high variability depending oil the Culture enrichment protocol. Since the metabolism for EPS production is rather difficult to define, it was not possible to develop a reliable estimation model based oil metabolic principles only. Instead, projection of latent structures (PLS) linear regression constrained by steady state carbon balance was employed. PHB concentration and biomass production rate were directly estimated by the PLS model, whereas acetate concentration was indirectly estimated through the carbon balance. The method was validated experimentally with data of four experiments carried out in a SBR. Accurate oil-line estimations were obtained with regression coefficients (r(2)) of 0.986 and 0.980 for biomass concentration, 0.976 and 0.999 for PHB and 0.992 and 0.999 for acetate concentration in calibration and validation, respectively. These results confirm the ability of the proposed methodology for on-line monitoring of the state variables in PHB production process by MMC. (C) 2008 Elsevier Ltd. All rights reserved.