The recent report of a superconductivity onset near the critical temperature
T
c
= 18 K in potassium-doped C
60
raises questions concerning the composition and stability of the superconducting phase. The effects of mixing and heat treatment of K
χ
C
60
samples prepared over a wide range of initial compositions on the superconducting transition was determined from shielding diamagnetism measurements. A single superconducting phase (
T
c
= 19.3 K) occurs for which the composition is K
3
C
60
. 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 Rb
χ
C
60
sample prepared and analyzed in an analogous way exhibited evidence for superconductivity with
T
c
= 30 K and a diamagnetic shielding of 7 percent could be obtained.
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