Abstract: In order to create a model to detect intracellular PHB (poly-β-hydroxy butyrate) rapidly and accurately, and to make timely predictions of the orthophosphate removal rate in a biological phosphorus removal system, infrared spectra were probed into the intracellular PHB transformation during denitrifying phosphorus removal. Correlation was established between PHB reduction and orthophosphate removal rate by means of both infrared spectra and gas chromatography. The results of the infrared spectrum analysis indicated that, in the denitrifying phosphorus removal system, an intracellular PHB characteristic peak was found at 1730 cm^-1, and the absorbance of PHB characteristic peak was positively correlated with the variation of PHB content. A model (PLS model) was established between the absorbance and PHB content through partial least squares; the correction and prediction coefficients of the model for the PHB content were 0.9630 and 0.9514 respectively, and the root mean square errors were 0.0310 and 0.0379 respectively. Based on the predictions, the correlation between PHB reduction and orthophosphate removal rate was established, with the coefficient 0.9790. The results showed that the PLS model established through infrared spectra and gas chromatography could characterized intracellular PHB variation rapidly and accurately in the biological phosphorus removal system and provide timely predications for phosphorus removal rate.