An amine-containing diacetylenic amphiphile was prepared from pentacosa-10,12-diynoic acid. Its monolayer properties as a floating Langmuir film were investigated. As a classic long chain amphiphile, condensed monolayers of pentacosa-10,12-diynylamine exhibited high collapse pressures and a solid-like character stable against compressive creep. A limiting molecular area of 23 Angstrom A(2)/molecule was found, consistent with other single chain diacetylenic amphiphiles. Isothermal compression studies showed a distinct transition in monolayer organization from a disordered to an ordered or solid film below a molecular area of 25 Angstrom A(2)/molecule. The nature of the film behavior at molecular areas greater than the transition to the solid could be manipulated by adjusting the subphase pH and/or the dissolved anion.