For pt.I see ibid., vol.16, p.L573 (1983). Reports magnetophonon oscillations in the two-dimensional electron gas formed in GaInAs at GaInAs-AlInAs heterojunctions. One series of oscillations is seen, corresponding to the energy of the degenerate 'InAs-like' LO phonon modes in GaInAs and AlInAs. There is also an interface phonon mode at the same energy. In bulk GaAInAs scattering is dominated by the 'GaAs-like' mode, and so as long-range phonon interaction between the electrons in the GaInAs and the interface and/or AlInAs 'InAs-like modes must be invoked to explain the dominance of scattering by these modes. Comparing the results with those for GaInAs-InP superlattices, it can be seen that changing the electrically inactive component of the heterostructure produces a radical change in the electron-phonon interactions.