This paper aims to set-up a compatibility criterion for couple of pectins at different degree of methoxylation (DM) based on specific viscosity data, slightly modifying a criterion already proposed in the field of synthetic polymers. Four different commercial pectins were preliminarily characterised by using dilute solution viscosity measurements and molecular weight was calculated from intrinsic viscosity data. Four ternary dilute solutions of pectin at different DM, were also investigated and two compatibility criteria, based on viscosity data, were applied. Data revealed that positive molecular interactions take place when pectins in solution have a different DM (i.e. HM/LM mixtures), whilst an incompatible behaviour was found for similar pectin in solution (i.e. both HM or LM). In addition, in the case of incompatible pairs of pectin, also the apparent average molecular weight of mixed pectin, obtained by measurements carried out on ternary solutions, showed an increase with respect to the additive mixing rule, owing to repulsive forces. On the contrary, owing to attractive forces in case of compatible pectin pairs, the apparent molecular weight is lower than the calculated average. (C) 2010 Elsevier Ltd. All rights reserved.