For the first time, a model for dynamic adsorption and tension of diffusion-controlled systems has been extended to binary premicellar mixtures of nonionic surfactants of different adsorption capacities and nonideal interactions in the monolayer. Local equilibria between subsurface concentrations and adsorbate densitities are modeled using the nonideal adsorbed solution theory, which describes these two elements. The model shows that larger molecules (those with the smaller adsorption capacity) tend to be preferentially adsorbed at low times, if they have equal adsorption equilibrium constants and diffusivities, and smaller moleules at longer times. This adsorption selectivity is reduced when larger molecules have a much larger adsorption equilibrium constant, or when there are negative deviations from ideality in the monolayer. This model's predictions are compared to tension data for two nonionic surfactants, C12E5 and Triton X-100, at 25°C. The data are represented well by the diffusion-controlled model with a finite diffusion-layer thickness, which describes the faster decrease in tension observed with the bubble surfactometer, compared to data with other techniques. With this model, surface coverages and concentration profiles are calculated, thus elucidating the adsorption selectivity of molecules of different adsorption capacities. Synergistic effect in dynamic tension and adsorption can be predicted. Mild synergism in dynamic tension lowering by the preceding nonionic surfactants is also observed.