The study of probes for CO sensing of a family of binuclear rhodium(II) compounds of general formula [Rh-2{(XC6H3)P(XC6H4)}(n)(O2CR)(4-n)]center dot L-2 containing one or two metalated phosphines (in a head-to-tail arrangement) and different axial ligands has been conducted. Chloroform solutions of these complexes underwent rapid color change, from purple to yellow, when air samples containing CO were bubbled through them. The binuclear rhodium complexes were also adsorbed on silica and used as colorimetric probes for "naked eye" CO detection in the gas phase. When the gray-purple colored silica solids containing the rhodium probes were exposed to air containing increasing concentrations of CO, two colors were observed, in agreement with the formation of two different products. The results are consistent with an axial coordination of the CO molecule in one axial position (pink-orange) or in both (yellow). The crystal structure of 3 center dot(CO) ([Rh-2{(C6H4)P(C6H5)(2)}(2)(O2CCF3)(2)]center dot CO) was solved by single X-ray diffraction techniques. In all cases, the binuclear rhodium complexes studied showed a high selective response to CO with a remarkable low detection limit. For instance, compounds 5 center dot(CH3CO2H)(2)([Rh-2{(m-CH3C6H3)P(m-CH3C6H4)(2)}(2)(O2CCH3)(2)]center dot(CH3CO2H)(2)) is capable of detection of CO to the "naked eye" at concentrations as low as 0.2 ppm in air. Furthermore, the binding of CO in these rhodium complexes was found to be fully reversible, and release studies of carbon monoxide via thermogravimetric measurements have also been carried out. The importance of the silica support for the maintenance of the CO-displaced L ligands in the vicinity of the probes in a noninnocent manner has been also proved.