This paper studies changes in reflective wavelength data caused by the action of fire. We consider the heterogeneity of fire effects in terms of the fraction of the observation that burned (f) and the combustion completeness (cc). A spectral mixture model and field and satellite observations of prescribed fires are used to examine the relationship between change in reflectance, and cc and f. The prescribed fires were lit in different South African savannah types during the SAFARI 2000 dry season campaign. Implications for the development of methods to retrieve cc and f, for the development of methods to map the spatial extent of fire-affected areas with known detection capabilities are discussed. Keywords: Surface heterogeneity, characteristics, Fire effects, Land process modelling, Management decision applications, Combustion completeness, Safari 2000 dry season campaign, Multi-temporal reflective wavelength data, Remote sensing, Imaging science, Photographic technology.