This study focused on quantitative chemical analysis of organic fouling of ultrafiltration membranes by protein (bovine serum albumin-BSA) in the presence of a polysaccharide (alginic acid-AA). Filtration experiments were performed in a dead-end cell with two microporous membranes. Prior to the filtration experiments, the ultraviolet-visible absorbance spectra of 140 solutions with various BSA and AA concentrations were used to calibrate a partial least square (PLS) model for estimating BSA concentration. This model, which had a validation error of less than 0.7 mg BSA/l, was used in the filtration experiments to estimate the concentration of BSA in the feed and permeate solutions, as well as in a solution containing the BSA fouling removed by simple water rinsing. These measurements, together with a mass balance of the different fractions of the filtration process, were used to estimate the reversible and irreversible fouling. Direct analysis of the fouled membrane was done by photoacoustic spectroscopy (FTIR-PAS), and a PLS model was used to estimate the amount of BSA present on the membrane. The fouling estimated from the FTIR-PAS spectra by PLS correlated well with the estimates from the mass balance approach, with typical errors of less than 10% of the range investigated. (C) 2010 Elsevier B.V. All rights reserved.