This paper treats the low energy rotation-vibration problem in CH5+, an extremely flexible molecule lacking a well-defined structure. Using SO(5) symmetry it determines zeroth order energies, and complete nuclear permutation S-5 symmetries, using a five-dimensional model involving rotation and two vibrations (which one might imagine as two different "cooperative" HCH bends). These two vibrations are presumed to be unhindered by the molecular potential function and their analytical form is not determined. The other ten vibrational degrees of freedom are presumed to be "rigid" (or averaged over). The general energy expression for this "rigid superrotor" is obtained as (B/2) [n(1)(n(1) + 3) + n(2)(n(2) + 1)] where B is the rotational constant and the non-negative integers n(1) and n(2) satisfy n(2) <= n(1). The superrotor predictions agree favourably with the available experimental data. Applications of the superrotor model to extremely flexible molecules other than CH5+ are discussed.(C) 2017 Elsevier B.V. All rights reserved.