The relatively simple and robust architecture of microelectronic devices based on carbon nanotubes, in conjunction with their environmental sensitivity, places them among the leading candidates for incorporation into ultraportable or wearable chemical analysis platforms. We used single-walled carbon nanotube (SWNT) networks to establish a mechanistic understanding of the solid-state oxygen sensitivity of a Eu(3+)-containing dendrimer complex. After illumination with 365 nm light, the SWNT networks decorated with the Eu(3+) dendrimer show bimodal (optical spectroscopic and electrical conductance) sensitivity towards oxygen gas at room temperature under ambient pressure. We investigated the mechanism of this unique oxygen sensitivity with time-resolved and steady-state optical spectroscopy, analysis of excited-state luminescence lifetimes and solid-state electrical transport measurements. We demonstrate a potential application of this system by showing a reversible and linear electrical response to oxygen gas in the tested range (5-27%).