In real-time monitoring of III-Nitride growth on patterned and masked substrates by spectroscopic reflectance, a characteristic interference pattern generated by the superposition of wave-fronts reflected at different pm-sized structures at the sample surface is measured. Up to now this time- and wavelength-dependent pattern was used only for empirical fingerprint-evaluation of III-Nitride growth processes which employ patterning or masking for bulk defect reduction. In this paper, we report on the analysis of real-time spectroscopic reflectance data measured in the range 1.65-4.5 eV during the epitaxial growth of GaN layers on structured Si(1 1 1) substrates. The successful implementation of a two-dimensional interference model into conventional thin-film analysis algorithms enables the quantitative analysis of characteristic vertical and lateral growth rates and overgrowth mechanisms involved. The new method is applied to optimize III-Nitride growth processes on patterned silicon substrates used for subsequent III-Nitride device growth. (C) 2004 Elsevier B.V. All rights reserved.