Here we use UV photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) to determine the valence electron structure of graphene oxide (GO) and hydrazine modified graphene oxide, or so-called reduced GO (rGO). We show that pristine GO has a low density of states (DOS) in the 2pπ region. Upon thermal treatment, under vacuum, to 200 °C, the DOS in the 2pπ region increase. The change in the DOS is also reflected in a change in the functional groups attached to the GO. These changes are followed by X-ray photoelectron spectroscopy (XPS). Based on the XPS measurements, the GO is described as highly carboxylated, much akin to pyromellite, with incorporated benzoquinone moieties. After heat treatment to 100 °C, we propose the close proximity of the pendant carboxyls undergoes a condensation reaction to form dianhdydrides such that the graphene sheet now consists of a mixture of dianhydride, phthalate-type carboxyl, and benzoquinone moieties with the work function increasing to 4.8 ± 0.1 eV. With further heating to 200-300 °C, these groups are cleaved and the work function decreases back to 4.2 ± 0.1 eV. Lastly, the rGO is shown to form pyrazoline-type bonds with an interrupted 2pπ graphene network which results in a decreased work function to 3.4 eV.