We investigated association of integral membrane protein–detergent particles induced by addition of polyethylene glycols or polyethylene glycol monomethyl ethers of different molecular weights. The extent of particle association was related to the energy level of particles in aggregates relative to that of isolated particles in solution. The energy difference is mainly determined by the compromise between the increase in the energy level of hydrophilic groups and the decrease in the energy level of hydrophobic groups on the particle surfaces. It follows that the polymers are more effective precipitants for more hydrophilic particles. The capacity per weight of polymers for solvent molecules (especially water molecules) increases with an increase in the molecular weight. For this reason, larger PEG(MME)s have stronger precipitation effects. In addition, the action of particle–particle depletion force of entropic origin, under which the decrease in the particles’ potential is small, accelerates the particle association.