AbstractSodium dodecyl 2‐hydroxy‐3‐methacryloyloxypropyl phosphate as an amphiphilic vinyl monomer was found to polymerize spontaneously without initiator in water. The polymerization takes place at a monomer concentration higher than the critical micelle concentration via a free‐radical mechanism, and complete conversion is achieved even at a very low initial monomer concentration (3 mmol/L) and a rather low temperature (35°C). In benzene, capable of forming inverse micelles, the polymerization also occurs spontaneously. In methanol, which gives an isotropic solution, the spontaneous polymerization proceeds only at high concentration (near 300 mmol/L). The 1H spin‐spin relaxation time (T*2) of the monomer changes discontinuously in a range of monomer concentration from 250 to 300 mmol/L in CD3OD, suggesting that some kind of monomer mutual interaction takes place in methanol too. Thus, it can be concluded that the micellar aggregation monomer molecules triggers the polymerization.