While carbon species are reasonably expected in Io's atmosphere, observational evidence does not yet support their presence in any measureable abundance. In this work, observations from the Cosmic Origins Spectrograph are combined over multiple orbits of the Hubble Space Telescope to search for C emission lines at far ultraviolet wavelengths. This technique is the most sensitive within supported HST modes, yet no emissions are found in the first 3 carbon charge states. The average C column over Io's disk does not exceed 3.4 × 1011 cm−2. Relative to sulfur, upper limits for C/S and C+/S+ are 0.002, while an upper limit for C++/S++ is 0.03. The C+/S+ ratio in the plasma torus at Io's orbital radius is ≤0.004. These upper limits are consistent with prior observational evidence, notably the C++/S++ of 0.00037 based on a tentative detection of C++ in the torus by the more sensitive Far Ultraviolet Spectroscopic Explorer (Feldman et al., 2004). Io's apparent lack of carbon is unique among the Galilean satellites and despite its widespread volcanism, carbon-to‑sulfur ratios Io's atmosphere are <1% of C/S in lunar rocks and chondritic samples, and < 10−4 of the solar photospheric abundance ratio. This fractionation is plausibly due to preferential loss of a lighter carbonaceous atmosphere compared to heavier sulfurous species that are recycled through Io's surface.