Mushroom-like Janus poly(methyl methacrylate) (PMMA)/poly(styrene-2-(2-bromoisobutyryloxy)ethyl methacrylate)-graft-poly(2-(dimethylamino)ethyl methacrylate) (PDM) particles synthesized in our previous report(27) were applied as particulate surfactants. The PDM moiety at the one side of the Janus particles reversibly exhibited the volume phase transition in response to pH and temperature in an aqueous medium; that is, the surface property of the Janus particles comprising both PDM and PMMA reversibly changed between amphiphilic and hydrophobic based on the nature of PDM. Consequently, 1-octanol-in-water emulsion droplets stabilized by the amphiphilic Janus particles coalesced in the alkaline region and at 60 degrees C around neutral pH because of desorption of the hydrophobized Janus particles from the interface to the oil phase.