A lifetime-based optical sensor for the measurement of dissolved and gaseous carbon dioxide has been developed. The basic principle is radiationless energy transfer from ruthenium(II)-4,40-diphenyl-2,20-bipyridyl as luminescent donor to thymol blue (a common pH indicator) as acceptor, both embedded in a hydrophobic matrix. In the presence of carbon dioxide thymol blue is protonated and changes its color from blue to yellow resulting in a decrease in the rate of energy transfer and consequently an increase in decay time. The decay time as a pCO2-dependent parameter was measured in the frequency domain with a blue light emitting diode as a light source modulated at 75 kHz. The present sensor displays a phase shift up to 168 in the range 0±30 hPa of pCO2 corresponding to a change in decay time from 0.38 to 1.05 ms. The detection limit was found to be 0.5 mM (22 ppb) of dissolved carbon dioxide and the response times are of the order of 15 s when going from 0 to 30 hPa pCO2. The temperature behavior of the sensor has been studied in detail and a linear relationship Arrhenius plot has been found. The effect of molecular oxygen as a potential quencher of the luminescence was investigated in detail. The carbon dioxide sensor is stable over weeks, and is robust.