-In capillary electrophoresis, the background electrolyte (BGE) composition is the key parameter for achieving the quantification of ions. This study is focused on the optimization of a BGE for anion separation (buffer, UV sensor, electroosmotic flow modifier and pH), and its application to whey anions. The quantification of chloride, sulfate, citrate and phosphate, was achieved with a relative standard deviation of about 1% for chloride and about 5---6% for the other anions, and compared to conventional one-by-one anion titration methods. © InralElsevier, Paris. capillary electrophoresis 1 whey 1 anion 1 background electrolyte Résumé -Optimisation de l'électrolyte support pour le dosage des anions par électro-phorèse capillaire: application au lactosérum doux. En électrophorèse capillaire, la composition de l'électrolyte support (BGE) est le paramètre clef pour quantifier les ions. Le but de cette étude est l'optimisation d'un BGE permettant l'analyse des anions (tampon, détection en UV-inverse, modificateur de flux électroosmotique et pH) et son application au dosage des anions du lactosérum. Le dosage quantitatif des chlorure, sulfate, citrate et phosphate est réalisé avec une répétabilité correcte (écart type relatif 1 % pour le chlorure et entre 5 % et 6 % pour les autres anions). La comparaison de l'électrophorèse capillaire et de méthodes de dosage des anions un par un par des techniques usuelles est également présentée. © InralElsevier, Paris. CE determination of whey anions had been previously published [16,17]. The analysis conditions were those of Jandik et al. [12], with the proprietary BOE from Waters.CE analysis of anions in human serum has been explored, but as serum proteins adsorb on the capillary walls and modify the migration times they were removed by ultrafiltration (molecular mass eut-off 3 000 g/mol) prior to injection. The best repeatabilities were obtained with a homemade BOE (5 mmol/L borie acid, pH 8 adjusted by NaOH, chromate and tetradecyltrimethyl ammonium bromide, TTAB) (RSD on peak height: chloride and sulfate: 3%, phosphate: 7%) compared to commercial BOEs (pyromellitic acid, NaOH, hexamethonium hydroxyde, triethanolamine, pH 7.7) [15].The quantification of phosphate by CE is not easy, because phosphate adsorbs on silica surface [9]. For synthetic mixtures, the repeatability on phosphate area with a chromate based electrolyte (pH 8) depends on the electroosmotic flow modifier (EOFM) used: RSD run to run was 5.4%, 2.7% and 3.0% with hexamethonium, hexadimethrine and tetradecyltrimethyl ammonium hydroxyde, respectively [7].The aim of this study was the optimization of the background electrolyte (buffer, EOFM, pH) for analysis of anions by capillary electrophoresis. Then application to the sweet whey anions (namely chloride, sulfate, citrate, phosphate and carbonate) was performed. CE titration of partially defatted sweet whey inc1uding proteins was compared with conventional titrations of anions, such as conductivity M. Rabiller-Baudry et al.