The isothermal crystallization of amorphous, vacuum condensed Er0.6Cu0.4 thin films was investigated in situ by transmission electron microscopy. Heterogeneous nucleation of ErCu crystallites was observed to occur on the thin rare-earth oxide layer which is inevitably formed on the external surface of the thin film exposed to the ambient atmosphere. The crystalline particles exhibited preferential growth in the direction parallel to the surface of the film. The crystallization process is interface controlled and characterized by a constant nucleation and constant growth rate. The kinetics of transformation were anslyzed in terms of Avrami's equation. The kinetic exponent n in Avrami's equation is equal to 2.9 in good agreement with the theoretical value for two-dimensional, interface-controlled growth. The experimental date allowed to derive the values of ΔE = 581 kJ.mole-1 for the overall activation energy of the crystallization reaction, Δcr = 151 kJ.mole-1 for the energy of critical nucleus formation and ΔEm = 143 kJ.mole-1 for the activation energy of atomic motion.