Kinetic Monte Carlo methods have been used to simulate structural transformations in fullerene layers during electrochemical intercalation with alkali-metal ions (A). Special attention is paid to the thermodynamic stability of the A(x)C(60) phases. The calculations point out a phase separation in the doped fullerene layer into alkali-metal-rich and alkali-metal-depleted areas at room temperature. The final state is represented by two phases which coexist as a stable fine mixture of nanoscale particles. The instability of homogeneous layers has potentially critical impact on their electrical properties and can explain the formation of nanostructures (20-50 nm) at the fullerene-electrolyte interface. Rb3C60 clusters are predicted to be larger than K3C60 ones for equal mean alkali-metal concentrations. Experimental data on electrochemical metal deposition on alkali-metal-doped fullerene substrates-in particular, atomic force microscopy measurements-are also consistent with the model proposed. (C) 2004 American Institute of Physics.