We present the first optical pH sensor with luminescence decay times longer than 1 μs. It is based on radiationless energy transfer from a luminescent ruthenium(II) complex as the donor to a colored pH indicator as the acceptor. The metal−ligand complex ruthenium(II) tris-4,4‘-diphenyl-2,2‘-bipyridyl was selected as the donor, and bromothymol blue and a reactive azo dye were selected as pH-sensitive acceptors. Strategies for co-immobilization of transition metal complexes and pH indicators into a hydrogel matrix were evaluated and discussed. pH transition intervals between 7 and 9 allow the measurement in the physiological range and also under marine conditions. Signal changes of up to 50% over 2.5 pH units are observed, depending on the respective acceptor content in the sensing film. Luminescence lifetime measurements of the sensing films were performed in the frequency domain with a blue LED as the light source. The effect of molecular oxygen (acting both as a quenching and a bleaching agent, thereby limiting the accuracy and the long-term stability of such sensors) was found to be crucial for practical applications. Since leaching of the acceptor also limits the stability of the sensing films, several strategies to reduce this effect were evaluated.