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Alginate is an anionic polysaccharide capable of forming insoluble particles with proteins. Hence, alginate has potential as a protein carrier. However, the role of physical properties of the polysaccharide, such as degree of polymerization (DP n ) and mannuronic/guluronic acid ratio, remains to be fully explored. Particle formation of a high and a low molar mass alginate (ALG) with β-lactoglobulin (BLG) at pH 2–8 depends on the average DP n (HMW-ALG: 1.59·10 3 ; LMW-ALG: 0.23·10 3 ) and the mannuronic/guluronic acid ratio (1.0; 0.6) as supported by using ManA 6 and GulA 6 as models. Dynamic light scattering (DLS) showed that particles of BLG with either of the two ALGs have essentially the same hydrodynamic diameter ( D H ) at pH 3 and 2, while at pH 4 particles of LMW-ALG/BLG have larger D H than of HMW-ALG/BLG. At pH 5–8 no significant particle formation was observed. ManA 6 did not form insoluble particles at pH 2–8, while GulA 6 formed insoluble particles, albeit only at pH 4. K d was approximately 10-fold higher for LMW-ALG/BLG than HMW-ALG/BLG and 3 orders of magnitude higher for an alginate trisaccharide/BLG complexation as determined by isothermal titration calorimetry (ITC). The alginate trisaccharide did not form insoluble particles with BLG at pH 3 and 4, though interaction still occurred. Δ H app and molar stoichiometry of BLG in the complexes with the two ALGs differed by a factor of 7, as did their DP n , which thus affected the interaction strength, but not the BLG content. At pH 4 the BLG content doubled in the particle due to BLG dimerization. The findings emphasize the importance of DP n , mannuronic/guluronic acid ratio and pH in formulations containing alginate/whey protein particles. [ABSTRACT FROM AUTHOR]