Fluids viscosified with viscoelastic surfactants (VESs) may have their viscosities reduced (gels broken) by the direct or indirect action of a synergistic internal breaker composition that contains at least one first internal breaker that may be a mineral oil and a second breaker that may be an unsaturated fatty acid. The internal breakers may initially be dispersed oil droplets in an internal, discontinuous phase of the fluid. This combination of different types of internal breakers break the VES-gelled aqueous fluid faster than if one of the breaker types is used alone in an equivalent total amount.