Abstract The stabilization of water/air, water/oil and water/solid interfaces by Janus particles with polystyrene (PS) and poly(methacrylic acid) (PMAA) hemispheres was systematically investigated. The stabilization of these interfaces is of relevance for the formulation of foams, emulsions and dispersions. The Janus particles were prepared from micellar solutions of polystyrene-b-polyisoprene-b-poly(tert-butyl methacrylate) (PS-PI-PtBMA) triblock terpolymers by selective cross-linking of the polyisoprene domain on a multi-gram scale, followed by hydrolysis of the PtBMA block. For the investigation of water/oil-emulsions a series of hydrophobic oils (paraffin oil, xylene, peanut oil, isopropyl myristate) with increasing polarity was selected. We found that the Janus particles do not stabilize the air/water interface and efficiently suppress foaming. The Janus particles showed a pronounced selectivity in their efficiency to stabilize w/o-emulsions. Isopropyl myristate water/oil emulsions could be stabilized up to several months with only 0.2 wt.-% of Janus particles. In general, interfaces with air and very hydrophobic oils and pigments were not stabilized, indicating that a certain polarity and interfacial mobility is necessary for an efficient stabilization by Janus particles. The Janus particles can be used as sole stabilizers in the soap-free emulsion polymerization of styrene/n-butyl acrylate or methyl methacrylate/n-butyl acrylate under commercially relevant semi-batch reaction conditions. Polymer coatings prepared from Janus particle-stabilized acrylate and styrene/acrylate dispersions showed a significantly reduced surface hydrophobicity as compared to surfactant-stabilized dispersions.