The present invention relates to a surfactant mixture comprising at least three ionic surfactants which differ in terms of the hydrocarbyl moiety (R1)(R2)—CH—CH2— and are of the general formula (I) where R1, R2, A0, k, X, o, Y, a, b, M are each as defined in the description and the claims. The invention further relates to the use and preparation thereof, and to aqueous surfactant formulations comprising the mixtures, and to processes for producing mineral oil by means of Winsor type III microemulsion flooding, in which the aqueous surfactant formulation is injected into a mineral oil deposit through injection wells and crude oil is withdrawn from the deposit through production wells.