Thin liquid films containing colloidal particles are considered to be the key structural elements of three-phase foams containing liquid, gas, and colloidal particles. This study is aimed at understanding the stability of such films in the absence of any surfactants. The particles form a layered structure in the film and produce a stepwise thinning in the thin liquid films. We report here for the first time the effects of particle concentration and size on film thickness transition of curved liquid films containing monodispersed colloidal particles. The rate of stepwise film thinning was observed to be high when particle concentration was low and both particle size and film size were large. The phenomenon of stepwise film thinning (i.e., stratification) is rationalized on the basis of diffusion of colloidal particles from the film to the meniscus, i.e., the diffusive osmotic mechanism. There exists a critical film size below which at least one layer of particles always stays in the film (i.e., black spot expansion does not occur). This critical size is dependent upon both particle size and concentration. Also, Monte Carlo simulations of the film show that, at a high particle concentration, better particle in-layer structure develops that increases the energy barrier, inhibiting particle diffusion from the film to the bulk meniscus. Copyright 2001 Academic Press.