The effects of high pressure gases (P less than or equal to 400 bar) on the R(II), R(I), and R(V) rotator phases of 21, 23, and 25 carbon normal alkanes were studied via x-ray scattering. We have measured the pressure and temperature dependence of the rotator structures and present these results in terms of the essential structural parameters : layer spacing, area per molecule, lattice distortion, and tilt. The pressure was generated by one of three gases : helium, nitrogen, or argon. In the rotator phases, argon and nitrogen intercalate between the layers while helium acts mostly as a noninteracting pressurizing medium. The thermal expansion and compressibility are anomalously large in the rotator phases, and this implies that the heat capacity in the rotator phases is dominated by anharmonic effects.