Ion-induced domain formation was investigated in mixed lipid monolayers using fluorescence microscopy techniques. Specifically, the mechanisms and time scales of two-dimensional solid domain growth and disappearance in fluid monolayers were investigated for mixed monolayers of anionic and zwitterionic phospholipids, which were either transferred to a solid support or "free" at the air-water interface. Experimental observations showed differing behaviors for the two systems above some threshold pressure. Monolayers at the air-water interface showed immediate formation of dendritic domains in the presence of calcium, with an average domain diameter of 20 mu m. These domains did not undergo further evolution with time, and their growth was reversible, as detected by reducing (reversing) the pressure to below the threshold value. In contrast, domains in films on a solid support exhibited a two-step growth mechanism; in the initial fast step, round, polydisperse domains of average size 10 mu m formed within minutes of deposition of the monolayer. These domains then grew via a much slower process, which appeared to be a diffusion-controlled two-dimensional Ostwald ripening, in which large domains grow at the expense of smaller ones, and were still evolving after 45 h. Growth and disappearance did not follow a reversible path, as domain disappearance upon removal of calcium was a uniform shrinkage in diameter, of all the domains, regardless of size, and was complete in 20 h. These results show that long-term equilibration or "aging" effects can occur in motionally restricted monolayers and bilayers.