1. The formation of density gradients with cesium oxalate and acetate in a preparative ultracentrifuge was studied and compared with Cs 2SO 4 and CsCl gradients. Cesium oxalate formed a gradient almost identical to CsCl, while acetate formed a very flat gradient, suggesting higher resolving power. 2. DNA (calf thymus), RNA (extracted from rat-liver ribosomes and poliovirus) and poliovirions were centrifuged to equilibrium in cesium oxalate and acetate; for each the apparent buoyant density was compared with their densities in Cs 2SO 4 and CsCl. The poliovirion had virtually the same density (1.33–1.34 g·cm −3) in all cesium salts. DNA showed increasing densities of 1.43, 1.51, 1.71 and 1.96 g·cm −3 in cesium sulfate, oxalate, chloride and acetate respectively. RNA could be banded in cesium oxalate at a density of 1.80 g·cm −3 (1.64 in Cs 2SO 4) but not in cesium acetate. 3. The introduction of cesium oxalate and acetate in density gradient work should give more flexibility to this technique because changed solvation of DNA and RNA together with different gradient characteristics may enhance the separation of closely related nucleic acids.