The highly excited vibrational levels of HCP exhibit a regular energy level and intensity pattern characteristic of 2:1 bend-CP stretch polyads. Stimulated by the experimental observation of vibrational levles with rotational constants (B-values) 5%-10% larger than other observed levels, Schinke and co-workers noticed that these large-B levels were characterized by atypical nodal structures indicative of large amplitude motion along the minimum energy HCP ↔ CPH isomerization path. In this paper, we show that the transition from “nomal-mode-type” to “isomerization” vibrational states arises naturally out of a traditional spectroscopic (algebraic) effective Hamiltonian polyad model. A global least squares fit, based on this polyad H^eff model, shows that all of the obseved “isomerization” states belong to polyads and that the eigenvectors of this H^eff model have the qualitatively distinct nodal structure first noticed by Schinke and co-workers. The “isomerization” states are not indicative of a breakdown of the polyad model; rather they are a natural consequence of this traditional spectroscopic model.