Thermal decomposition of graphene oxide (GO), often referred to as “thermal reduction” is broadly used to obtain so-called “thermally reduced GO”. At the same time, chemical and structural transformations, accompanying this process remain largely unexplored. In this work, using the combination of electron spin resonance spectroscopy, thermogravimetry, IR spectroscopy, and X–ray powder diffraction, we investigate the early stages of the GO thermal decomposition, which occur in the 80◦C–190◦C temperature range. Massive decomposition of the oxygen-containing groups begins at ∼130◦C. At this temperature we observe formation of C-H bonds and a sharp increase in the content of paramagnetic centers. The highest content of the radicals 1.3 × 1018 spin/g is registered in the samples, annealed at 150◦C. This is 3.5 times higher than that in original GO (3.8 × 1017 spin/g). At the same temperature we observe the loss of the interlayer registry in the material due to crumpling of the partially decomposed GO layers, and the C–H bonds are no longer observed. At 190◦C, the content of the paramagnetic centers sharply decreases down to 1.0 × 1017 spin/g, being 3.8 times smaller than that in original GO. This suggests that electrons are largely delocalized due to the enlargement and percolation of graphenic domains, and/or dangling bonds, formed at 130–150◦C largely recombine. Our new findings add critical details to understanding the fine chemical structure and chemistry of GO.