AbstractThe efficiency of size separations of polystyrene sulfate, 120‐1085 nm radius, by molecular sieving in polymer solutions, expressed by the separation efficiency functionS=MOdKR(R)/dRTe−KR(R)T[whereMOis the mobility in free solution,KRthe retardation coefficient,Rthe geometric mean radius of the particle,KR(R) the retardation coefficient as a function ofR, and the polymer concentration isT] increases from methylhydroxypropyl cellulose to polyvinyl alcohol to uncrosslinked polyacrylamide to agarose above its gelling temperature. Separations of DNA, in the size range of 3–21 nm radius, are by at least one order of magnitude more efficient than those of polystyrene sulfate in the size range of 120–1085 nm radius. A plot ofS vs. Rin the experimental range ofTis constructed for the four polymer solutions; this allows one to select optimal media and concentrations for the sieving of particles in the desired range of molecula