A new family of robust, non-toxic, water-compatible ruthenium(II) vinyl probes allows the rapid, selective and sensitive detection of endogenous carbon monoxide (CO) in live mammalian cells under normoxic and hypoxic conditions. Uniquely, these probes incorporate a viscosity-sensitive BODIPY fluorophore that allows the measurement of microscopic viscosity in live cells via fluorescence lifetime imaging microscopy (FLIM) while also monitoring CO levels. This is the first example of a probe that can simultaneously detect CO alongside small viscosity changes in organelles of live cells. Carbon monoxide (CO) has long been associated with its toxicity, however, this colourless and odourless gas also plays a key role in cellular messaging. [1] Its anti-inflammatory, antiproliferative, anti-apoptotic and anti-coagulative properties are now recognised. Intriguingly, under pathophysiological conditions (e.g., inflammation) CO production in cells increases, [2] and the real-time monitoring of these changes could potentially provide diagnostic information. Haemoglobin is required as a substrate for CO production in vivo and the haem oxygenases (HO-1 and HO-2) play a key role in the generation of this gas in mammals. Emerging evidence suggests that the increased generation of HO-derived CO plays a critical role in the resolution of inflammatory processes and alleviation of cardiovascular disorders, [3] which has driven interest in CO-releasing molecules (CORMs) for therapy. [4] A major obstacle is the lack of effective methods to track CO in biological systems in real time. [5] Imaging with emissive probes has emerged as one of the most powerful techniques to detect biologically important molecules. However, designing selective CO probes for the cellular environment is challenging, with its wide range of reactive species and variation in pH. He [6] and Chang [7] pioneered two very different