Several highly emissive, crystalline salts (ClO4-, PF6-, BF4-, B(C6F5)4-, and tfpb-; B(C6F5)4- = tetrakis(pentafluorophenyl)borate; tfpb- = tetrakis(bis-3,5-trifluoromethylphenylborate) of the Ru(pp)32+ (pp = bpy (2,2‘-bipyridine), phen (1,10-phenanthroline) or 4,7-Me2phen (4,7-dimethyl-1,10-phenanthroline) lumophore have been tested as oxygen sensors. Oxygen detection by luminescence quenching correlates with the void space in the crystalline lattice, particularly in the case of Ru(phen)32 which has channels occupying approximately 136 Å3 per Ru in the crystals. The emission intensity and lifetime quenching of Ru(phen)32 displayed strictly linear (R2 = 0.9996) Stern−Volmer behavior (plots of I0/I and τ0/τ vs mole fraction of oxygen) with a slope of 2.43. A single exponential (τ = 640 nsec, pure nitrogen; 190 nsec, pure oxygen) is observed for the emission intensity decay for all oxygen concentrations. The time dependence of the emission caused by a step function air pressure drop is significantly affected by changing the light penetration depth when different excitation wavelengths are used (at 400 nm, t1/2 = 120 ms; at 518 nm, t1/2 = 2200 ms). These experiments are consistent with the diffusion of oxygen molecules in and out of Ru(phen)32 crystals with a diffusion coefficient on the order of 10-7−10-8 cm2/s. The technological significance of these crystalline oxygen sensors was demonstrated by long-term stability studies and by the successful calibration of a ballprobe sensor coated with crystalline Ru(phen)32 against a dissolved oxygen Clark electrode using a partial least-squares (PLS) model with a single principle component.