The phase diagram of dioctadecylamine Langmuir monolayer has been determined from pressure-area isotherms and from direct observations of the monolayer using Brewster angle microscopy. We describe the observed domains, textures, and phase boundaries for the mesophases presented by this system between 5degreesC and 45degreesC, at pH=3. The phases were named as G, LE, S-1, and S-2 according to their apparent texture. Contrast between domains of condensed phases is quite low, indicating small tilting of the amine tails. The G-S-1, G-S-2, G-LE, and LE-S-1 transitions seem to be of first order; meanwhile, the S-1-S-2 transition seems to be of second order. We observed how the froths changed when the area density increases at constant pressure, along the G-L, G-S-1, and G-S-2 phase transitions. In the L-S-1 transition, the shape of the domains of the emerging phase is of sixfold dendrites. The phase diagram obtained is very different from those obtained for single chain amphiphiles. In this system, the extent of the phases depends on the acid used to fix the pH. In addition, all our observations indicate that dioctadecylamine does not form a monolayer above pHsimilar to3.9. (C) 2003 American Institute of Physics.