We use imaging Fabry-Pec rot observations to explore the dynamical conditions in four southern ultra- luminous infrared galaxies. These galaxies all show morphological and kinematic fea- (L IR Z 1012 L _ ) tures indicative of a major merger, but they span a wide range of inferred dynamical ages: the youngest system is just past the -rst encounter, while the oldest has already merged into a single object. This diversity in dynamical states indicates that ultraluminous activity is not con-ned solely to late-stage mergers. The Ha emission is more spatially concentrated in the later stage mergers, in agreement with models that show that mergers drive gas inward toward the nucleus, but high luminosities are achieved even in young interactions where the Ha is quite extended. Our data indicate that physical details other than the dynamical phaseEsuch as the internal structure or gas content of the galaxiesEcan play a strong role in determining the luminosity evolution of merging galaxies. We also -nd evidence for massive star-forming complexes at large radius in the tidal debris, which reaffirms the notion that some dwarf galaxies may be spawned during the merger of more massive disk galaxies. Subject headings: galaxies: active E galaxies: interactions E galaxies: kinematics and dynamics E galaxies: starburst E infrared: galaxies