N- methyl- N- acylglucamines are suitable as thickening agents in aqueous surfactant solutions containing one or more anionic surfactants of the group including alkyl ether sulfates and alkyl sulfates, the N- methyl- N- acylglucamines containing at least 60 wt.- % of N -methyl -N -acylglucamines that have a Cn-, Cu- acyl group or an unsaturated Cis- acyl group and at the same time less than 5 wt.- % of N- methyl- N -acylglucamines that contain an acyl group < C12. The surfactant solutions are particularly used in cosmetic compositions.