Brewster angle microscopy (BAM) was applied to a study of temperature-induced phase transitions in Gibbs monolayer films of two long-chain alkanols (octadecanol and 1,1,2,2-tetrahydroperfluorododecanol) at the hexane/water interface. Changes of the interfacial tension along the experimental temperature scans were monitored simultaneously with the BAM micrographs by quasielastic light scattering from the interface. In this way it was possible to locate the phase transition temperature T-t in situ from the observed break. in the temperature derivative of the interfacial tension. Domains of the condensed phase are seen below T-t, and the morphological features of these domains show remarkable similarities with those observed in Langmuir films at the free surface of water. The coexistence of the condensed phase with a dilute (gaslike or expanded) phase is not limited to a single temperature (as expected for a first-order phase transition) but extends over a temperture range of ca. 15 K below the transition temperature. This finding is attributed to surface-active impurities which will accumulate in the dilute phase of the interfacial film.