A bending modulus of surfactant monolayers in a microemulsion system composed of AOT, D2O and deuterated decane were investigated by means of neutron spin echo. In this system, a water‐in‐oil droplet structure at ambient temperature and pressure decomposes into two‐phases both with increasing temperature and pressure. However, the bending modulus slightly decreased with increasing temperature while it increased with increasing pressure. This behavior was explained in terms of a microscopic model proposed by Wu¨rger (Phys. Rev. Lett.85(2000) 337). It was confirmed that an increase of an interaction between hydrocarbon tails of AOT molecules with increasing pressure could be an origin of the pressure‐induced phase transition. © 2004 American Institute of Physics