A study has been made to try to elucidate the locus of polymerization and mechanism of an interfacial polycondensation between sebacoyl chloride and hexamethylenediamine. It is concluded that polymerization occurs in a mixed monolayer of the adsorbed monomers. When the interfacial pressure of the adsorbed mixed monolayer exceeds the equilibrium spreading pressure of the polymer, the latter is precipitated from the monolayer, giving rise to a thick film at the interface. The interfacial pressures above which precipitation occurred were measured for the interfaces carbon tetrachloride/water, n-heptane/water and benzene/water and found to be 14.5, 9.4 and 6.5 dynes cm–1 respectively. The high rate constants and the high and specific molecular weight ranges generally found in this type of polycondensation are discussed on the basis of a two-dimensional reaction.