In this paper, we analyze the Brownian motion of ceramide-enriched condensed domains immersed in a fluid sphingomyelin-enriched monolayer at the air-water interface. The diffusion coefficient of the domains is determined under different molecular packings and domain arrays, and the monolayer viscosity is calculated. With this approach, the effect of domain crowding and of intrinsic monolayer viscosity can be split. We found that, for mixed monolayers of palmitoylated sphingomyelin and ceramide, the monolayer viscosity depends on the lateral pressure and oil the domain-domain distance. In a 9:1 proportion of sphingomyelin: ceramide, the viscosity is about 4 x 10(-9) N s m(-1) below 15 mN m(-1) (continuum sphingomyelin phase in a liquid expanded state) and increases at higher lateral pressures (continuum phase in a condensed state). The viscosity change with pressure is caused by both an increase of intrinsic viscosity and ail increase in domain crowding. At very high domain crowding, the monolayer viscosity increases because the domains are held in place by steric hindrance generated by the other condensed domains or the array. These are short-range forces, effective when the domains are close together. As the domain array is relaxed and the domain-domain distance increases, these forces become negligible, and repulsive dipolar interactions appear to acquire importance. For the lipid mixture analyzed, the dipolar repulsion is more noticeable on subphases of 0.15 M NaCl than on pure water, revealing the influence Of Surface electrostatics.