In this paper, we report a comparison of the thermal decomposition pathways of ultrathin oxide layers, formed on Ge(100) and Si(100) surfaces, as revealed by photoemission studies, employing synchrotron radiation. The oxide layer, in both the cases, consists of a mixture of, mainly, monoxide and dioxide species. On annealing, both the oxides undergo thermal decomposition and the desorbing species is the corresponding monoxide. However, we find that their decomposition pathways are entirely different. On annealing the Ge oxide layer, the GeO2 species transforms to GeO on the surface and finally desorbs from the surface at similar to 425 degrees C. In contrast, annealing results in the transformation of SiO to SiO2 up to a temperature, lower than the desorption temperature (similar to 760 degrees C). At higher temperatures, SiO2 possibly transforms back to SiO and subsequently desorbs. These phenomena may be related to the interface properties, since the thermodynamic properties of individual oxides predict an opposite trend in their thermal stability.