We have measured specular neutron reflectivity for dimyristoylphosphatidylcholine (DMPC) films spin-coated on Si substrates with and without NaI. The obtained reflectivity profiles were analyzed using Parratt's equation with taking the distribution of the stacking number of bilayers into consideration, since the bilayer stacking still remains on the film surface and it forms terrace-like islands. The repeat distance of the bilayer stacking for the DMPC/NaI mixture film was drastically decreased at 80 degrees C, and the evaluated value was consistent with that in an "interdigitated gel" phase. Since this change was not observed for the pure DMPC film, we concluded that the phase transition to an "interdigitated gel" phase was induced by the addition of NaI. (c) 2007 Elsevier B.V. All rights reserved.