Epitaxial layers of GaN were grown by molecular beam epitaxy on GaAs (100) and Si (111) substrates. Nitrogen was provided from a rf-plasma source while elemental Ga was evaporated from a Knudsen cell. The growth process was performed in an ultrahigh vacuum chamber that is optically aligned with a multichannel Raman spectrometer. This setup allows Raman spectra to be taken online, i.e., during the growth process. Utilizing resonant Raman scattering conditions, spectra with a sufficient signal-to-noise ratio were taken even at the growth temperature of 600 degrees C. For both substrates the evolution of compressive strain at the interface was monitored from the frequency shift of the substrate phonons in the initial phase of the growth process.