Abstract EPR and MMMA methods revealed two K 3C 60 phases, the unstable one with T c=21.0 K containing C 1− 60 centers and a stable one with T c=18.0 K constituting of C 60 molecules in the form of C 3− 60 ions. The metastable phase appears only in the first stage of potassium intercalation of fullerene at the C 60/K 3C 60 phase boundary. This phase is being transformed to a stable one on further doping to K 3C 60. The appearance of a metastable phase is related to the instability of the fullerene C 60 fcc structure during the process of octahedral sites being filled by potassium ions. The critical temperature T c shift upwards from 18.5 to 21.0 K is ascribed to the possible process of potassium transient back charge transfer valency, i.e. K 3 1+C 60 3−↔K 2 1+K 1−C 60 1− [3K 1++C 3− 60⇔2K 1++K 1−+C 60 1−], which could enlarge the unit cell necessary to shift a superconducting critical temerature according to T c on volume dependence.