Abstract. We study the compressible and incompressible two-phase flows separated by a sharp interface with a phase transition and a surface tension. In particular, we consider the problem in R N , and the Navier-Stokes-Korteweg equations is used in the upper domain and the Navier-Stokes equations is used in the lower domain. We prove the existence of R-bounded solution operator families for a resolvent problem arising from its model problem. According to Shibata [13], the regularity of ρ + is W 1 q in space, but to solve the kinetic equation:] on Γt we need W 2−1/q q regularity of ρ + on Γt, which means the regularity loss. Since the regularity of ρ + dominated by the Navier-Stokes-Korteweg equations is W 3 q in space, we eliminate the problem by using the Navier-Stokes-Korteweg equations instead of the compressible Navier-Stokes equations.