Abstract: We find evidence for a strong thermal inversion in the dayside atmosphere of the highly irradiated hot Jupiter WASP-18b (T(sub eq) = 2411 K, M = 10.3 M(sub J)) based on emission spectroscopy from Hubble Space Telescope secondary eclipse observations and Spitzer eclipse photometry. We demonstrate a lack of water vapor in either absorption or emission at 1.4 µm. However, we infer emission at 4.5 µm and absorption at 1.6 µm that we attribute to CO, as well as a non-detection of all other relevan…