The melting point of symmetrical quaternary ammonium salts (CnH2n+1N+ X(-) exhibits a sudden drop as n, the alkyl chain length increases. The location of this discontinuity varies with the nature of the associated anion. Concommitant with the drop in melting point is a towering of the space group symmetry. An explanation based upon relative cation and anion sizes is proposed. Chickos’s method for estimating the fusion energetics of solids has been adapted to handle "hydrocarbon-like" large quaternary cations. Extrapolation to small cations and comparison with melting points yields an estimate of the energy of the crystal structure change responsible for the melting point discontinuity.