It is well known that surfactants and amphiphilic copolymers commonly used to stabilize emulsions can often be substituted, as emulsifiers, by solid colloidal particles. Long after their original discovery, particle-stabilized emulsions ("Pickering emulsions") are currently attracting renewed interest for their potential role as precursors in the fabrication of capsules for drug delivery and other applications. Most recent studies have focussed on emulsions stabilized by particles alone, and differences as well as analogies between particulate emulsifiers and surfactants have been pointed out. In this presentation we report surprising cooperative effects of particles and surfactants in mixed emulsifier systems. Our experimental observations suggest that standard cosurfactants play a very different, less intuitive role in Pickering emulsions than they do in ordinary, surfactant-stabilized emulsions.