Polymerized ionic liquid (PIL) nanoparticles are for the first time applied as sole stabilizers in aqueous emulsion polymerization and reveal an astonishing and unexpected behavior. In a well-dispersed state, the PIL nanoparticles serve as an unexpectedly effective stabilizer for polystyrene dispersions, enabling solids content of greater than 40%. However, the same PIL as dry powder is hydrophobic and, in accordance with Bancroft's rule, unable to stabilize aqueous dispersions. This ambivalent behavior of PIL is extremely beneficial for the application of aqueous dispersions because, as desired for decades, the hydrophilic dispersed state during synthesis turns hydrophobic in the dried state during application of the polymer.