In this study, we discuss the origins of decomposition of the glassy phase in Al-rare earth melt-spun amorphous alloys. The small angle X-ray scattering results indicate that the amorphous phase undergoes phase separation by spinodal mechanism prior to devitrification. We recall a concept of Hume-Rothery of liquid immiscibility in binary liquid systems with strongly negative enthalpy of mixing. It is suggested that the experimental results may be explained by existence of an inflection in the Gibbs free energy curve of the amorphous phase due to effect of varying atomic packing efficiency with composition.