Stable concentrated solutions of basic dyestuffs of the formula ##STR1## wherein R denotes the remaining part of a 5- or 6-membered heterocyclic ring,R.sub.1 denotes optionally substituted alkyl, alkenyl, cycloalkyl, aralkyl or aryl,R.sub.3 denotes alkyl andA denotes an aromatic or heterocyclic radicalare prepared by reacting bases of the formula ##STR2## with dialkyl sulphates of the formula(R.sub.3 -O-).sub.2 SO.sub.2in organic solvents which are completely or partially miscible with water and which do not react with the dialkyl sulphates or react with these more slowly than the dye bases.