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The aim of this study was to characterize the influence of 3-(N-morpholino)-propane-sulfonic acid (MOPS) on alginate production by Azotobacter vinelandii and its chemical composition (particularly its acetylation degree), as well as on the rheological behaviour of alginate-reconstituted solutions.Cultures were grown in 500-ml flasks containing 90 ml of medium supplemented with MOPS in concentrations ranging from 0 to 13·6 mmol l−1. The acetylation degree of the alginate was significantly influenced by the MOPS concentration, obtaining an alginate with an acetylation degree of 1·4% when 13·6 mmol l−1 of MOPS was added to the medium. This value was twice as high as that obtained when no MOPS was used. The higher acetylation of the polymer resulted in higher viscosity of alginate solutions, having a more pronounced pseudoplastic behaviour.MOPS added to the culture medium determines the acetyl content of the alginate and thus, the physico-chemical properties of the polymer.These changes in the functional properties of the polymer can be very valuable in specific applications of alginate in the food and pharmaceutical fields.