The recent report of a superconductivity onset near the critical temperature Tc = 18 K in potassium-doped C60 raises questions concerning the composition and stability of the superconducting phase. The effects of mixing and heat treatment of Kx C60 samples prepared over a wide range of initial compositions on the superconducting transition was determined from shielding diamagnetism measurements. A single superconducting phase (Tc = 19.3 K) occurs for which the composition is K3C60. The shielding reaches a maximum of greater than 40 percent of the perfect diamagnetism, a high value for a powder sample, in samples prepared from 3:1 mixtures. A RbxC60 sample prepared and analyzed in an analogous way exhibited evidence for superconductivity with Tc = 30 K and a diamagnetic shielding of 7 percent could be obtained.
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