This is a continuation of our earlier work aimed at predicting the millimeterwave spectrum of protonated methane CH 5 þ . As for protonated acetylene C 2 H 3 þ , it is the millimeterwave spectrum that will most directly provide the experimental information needed to understand the large amplitude motion of the molecule. Literature ab initio calculations show that the large amplitude motion of the five protons around the central carbon nucleus in CH 5 þ is not completely free, but is restricted by potential barriers at the trigonal bipyramid ðD 3h Þ; square pyramid ðC 4v Þ and end-on-H 2 ðC 3v Þ forms. Thus, the large amplitude motion proceeds mostly in the coordinate space that connects the structures called C s ðIÞ; C s ðIIÞ and C 2v : These structures have essentially identical electronic energies and very similar rotational constants, as has already been shown in the literature. We calculate that they also have very similar dipole moments. The topology of the space of the large amplitude motion that connects the 120 versions of the C s ðIÞ structure, the 120 versions of the C s ðIIÞ structure, and the 60 versions of the C 2v structure is considered here. The spectral signature of this large amplitude motion in the rotational spectrum is calculated with absolute intensities. It is hoped that these results will aid and stimulate attempts to see and assign the high resolution gas phase millimeterwave absorption spectrum of CH 5 þ . The J ¼ 1 ˆ0 spectrum is predicted to be centered in the region 220-235 GHz, and if all the large amplitude motion splittings of this line are resolved, the strongest component (the K i ¼ 0 ˆ0 line) is predicted to have an integrated absorption intensity of 13 m/mol at 77 K.