The aim of the present study was to verify whether magnesium-induced increase of barium coating of the colonic mucosa is specifically due to the increase of barium suspension viscosity. One hundred sixty-one patients scheduled for double-contrast barium enema (DCBE) were randomised in one group of 23 patients (control group, CG), and three groups of 23 pairs (G1, G2 and G3), i.e. 138 patients. The DCBE was performed with standard technique using a barium suspension with dynamic viscosity of 280 cPs (CG), or with viscosity increased to 320 cPs (G1), 2500 cPs (G2) or 3200 cPs (G3), by extemporaneous addition of MgSO4 1, 2 or 3 g in one member of the pair, Na2SO4 1, 4.2 or 8 g in the other one. Three radiologists evaluated on an ordinal scale mucosal coating and free fluid. In all magnesium subgroups barium coating was better than in CG (p < 0.008), the highest value being obtained in G2. No difference was observed as regards free fluid. Inside paired groups, mucosal coating was more effective in magnesium than in sodium members (p < 0.0002). Viscosity of barium suspension being equal inside each group, this effect must be attributed to magnesium-specific interactions with additives of the barium suspension inducing events other than viscosity changes.