WASP-12b dayside emission spectrum with atmospheric models. Our comprehensive analysis of available WASP-12b data includes HST/WFC3 points (green squares) in the NIR, Spitzer/IRAC points (red circles) from 3 – 10 μm, and previously-published ground-based points (triangles). The isothermal blackbody model is rejected at 4.5 μm with a significance of 7σ. The oxygen-rich model requires 5x less H2O and 100x more CO2 than solar composition. This physically-implausible scenario is 670x less probable than the best-fitting carbon-rich model, which includes absorption due to C2H2 and HCN. We conclude that a dayside atmospheric model with a carbon-to-oxygen ratio > 1 provides the best fit to the available data.