Abstract The stability of a colloid has been treated in terms of repulsion by electric charge and attraction by cohesion of particles, but colloid particles are v-times larger in volume-size than the solvent molecules. v is as large as 103-6 and the molar energy of cohesion H/v is negligibly small compared with the electric repulsion energy Wel/v. Cohesion acts over the distance of 1—5 Å or only at the contact point. H does not increase with v, contrary to the DLVO theory. The molar energy of sedimentation Wsed/v is taken as an influential factor for the stability of a colloid of large particles. It is calculated from Stokes law as Wsed/v = Δρ u2, where Δρ is the density difference and u is the stationary velocity of sedimentation equal to (Δρ/η)(2/9)gr2, where η, r, and g are viscosity of solvent, radius of particle, and acceleration constant of gravity, respectively. Sedimentation occurs when Wsed/v is equal to kT/3, i.e., the perpendicular component of the energy of Brownian motion. The critical radius rc is rc = 30C nm, where C = 10 (η2/Δρ3)1/4 and k is Boltzmann's constant. rc is 10—100 nm for inorganic sol, above 100 nm for organosol and aerosol. A theory of colloidal solution is proposed by using a lattice model of size v. Molar free energy of mixing ΔG of the volume fraction of particles φ is ΔG = [(1-φ) ln (1-φ) + (φ/v) ln (φ/v)] R T + (φ Wsed-φ2Wel + φ2H)/v. Solubility is φ = [1- (r/rc)]/[1 + (1/v2) + (H-Wel)/v RT]. Wsed is absorbed internally in the dispersion system. When r > rc, sedimentation takes place to form muddy mixtures including agglomerates of particles. Cohesion acts only at a contact point and is negligibly small.