The aim of the present study was to verify whether the presence of magnesium in the colon lumen at the time of the double-contrast barium enema (DCBE) examination changes the quality of barium mucosal coating. The two members of 38 pairs of patients undergoing DCBE with a standardised technique were randomly subjected to bowel preparation with sennosides and magnesium sulphate, or sennosides and sodium sulphate. Mucosal coating, residual fluid and colon cleansing were assessed independently by three radiologists. The null hypothesis was tested by means of Wilcoxon's signed-rank test. Barium mucosal coating was judged to be better in the members to whom magnesium sulphate was administered (p = 0.0007). There was no difference in the amount of residual fluids (p = 0.3198). Colon cleansing was judged to be better in the members to whom sodium sulphate was administered (p = 0.0166). These results demonstrate, in a simple way, that magnesium ions increase barium coating of the colon mucosa in vivo. The underlying mechanisms (increase in viscosity of barium suspension through water subtraction owing to the hydrophilism of magnesium ions, or interactions with the polysaccharide additives) need further investigation. A first clinical application could be the integration of magnesium ions in a newly designed isotonic electrolyte solution containing polyethylene glycol for the oral colon wash-out.