The chain-length-dependent conformational transformation and melting behavior of triblock compounds alpha-hexyl-omega-hexyloxyoligo(oxyethylene)s, H(CH2)(6)(OCH2CH2)(m)O(CH2)(6)H (abbreviated as C(6)E(m)C(6)s) (m = 1-7), have been studied by differential scanning calorimetry and infrared spectroscopy. With an increase in the number of oxyethylene units (m), the molecular form of the triblock compounds in the solid state changes from an all-trans planar form (gamma form) to a planar/helical/planar triblock form (beta form) at m = 5. The melting points of the gamma form C(6)E(m)C(6)s are much lower than the melting points of n-alkanes with similar molecular masses. This result is interpreted as due to the higher Gibbs energy of crystal for C(6)E(m)C(6)s. The observed thermodynamic quantities indicate that the planar structure of the oligo(oxyethylene) chain is stabilized by the force of the magnitude that maintains the rotator phase of n-alkanes. The beta-form C(6)E(m)C(6)s melt stepwise through a solid-solid transition, at which the melting of only the alkyl blocks of the molecule occurs. The alkyl chains in the beta-form C(6)E(m)C(6)s are loosely packed in the crystal even below the solid-solid transition temperature, while the crystallinity of the oligo(oxyethylene) block is significantly high. The molecular form of the alkyl/oligo(oxyethylene)/alkyl triblock compounds in the solid state is determined by the balance of the intramolecular conformational restoring force in the central oligo(oxyethylene) block and the intermolecular packing force in the end alkyl blocks. The conformational restoring force results from a tendency of the oligo(oxyethylene) chain to resume its intrinsic helical structure.