Phase behavior studies and small-angle neutron scattering measurements of nonionic lamellar phases that contain dispersed colloidal particles show a strong interplay between semiflexible bilayers and particles. Addition of charged silica particles to lamellar phases formed with either mixtures of the nonionic surfactant n-dodecylpentaoxyethylene glycol monoether (C12E5) or C12E5 and hexanol (C6E0) in water changes the temperature of the phase transition from the single lamellar phase to its adjacent two-phase region. Close to the ternary phase boundary, particles weaken the long-range correlation between the highly undulated surfactant bilayers, as evidenced from the changes in the structure factor of the bilayers measured in the small-angle neutron scattering experiments. A self-consistent thermodynamic model is developed based on the free energy contributions of the binary interactions describing the phase behavior of the C12E5-water-colloid mixture. This model shows the effect of concentration, temperature, particle size, and bilayer rigidity on the phase behavior, and agrees well with the measurements.