The free-jet millimeter-wave absorption spectrum of 1,1-difluoroethane-Ar is reported. Most of the measured lines an split due to internal rotation of the methyl group and the tunnelling motion of Ar connecting two equivalent potential energy minima. The Ar atom, close to the CHF, group, eclipses one of the methylic hydrogens in the symmetryless geometry of the complex, reducing in this way the barrier to the internal rotation of the methyl group with respect to isolated 1,1-difluoroethane. For high J levels the distance of Ar from the molecule increases, however, due to the centrifugal distortion, and the barrier increases towards the value for 1,1-difluoroethane.