The thermal reactivity of NH3 with Si(001), Ge(001), Si1-xGex(001), and thinly Ge-covered Si(001) or Si-covered Ge(001) surfaces has been studied by means of in situ x-ray photoelectron spectroscopy in a temperature domain (T approximately 600-degrees-C) compatible with the usual growth of Ge-Si based heterostructures. A very marked difference between Si(001) and Ge(001) initial sticking coefficients is found, the latter surface being totally inert against nitridation by NH3 in contrast with the Si(001) surface. This nitridation selectivity provides easy access to information about surface termination in different situations where Si and Ge are potentially mixed, the reactivity depending on whether Si appears at the reactive interface or not. As a test, comparing the nitrogen uptake curves in the early stages of NH3 exposure, for all the studied structures except Si(001), a Ge-like initial sticking coefficient or a non-nitriding behavior is found. This is fairly explained on the basis of the well-known concepts of the Stranski-Krastanov growth mode for the Ge/Si system and by Ge surface segregation for the Si1-xGex and Si/Ge systems. Our result also implies the need of plasma-assisted treatments in order to achieve simultaneous Si and Ge nitridation of SiGe alloys.