A composition in the form of an injectable aqueous solution, the pH of which is between 6.0 and 8.0, includes at least a basal insulin, the isoelectric point pI of which is between 5.8 and 8.5 and a dextran substituted by radicals carrying carboxylate charges and hydrophobic radicals. Single-dose formulations at a pH of between 7 and 7.8 includes a basal insulin whose isoelectric point is between 5.8 and 8.5 and a prandial insulin.