A fluorescence microscopy technique has been developed that allows investigation of the luminescence properties of working film-based O-2 sensors with spatial resolution of < 5 m The new technique has been applied to investigate the properties of two sensor formulations that differ widely in their Stern-Volmer (SV) luminescence intensity response to variation in air pressure (P-air). One sensor is based on platinum(II) meso-tebrakis(pentafluorophenyl)porphyrin (PtTFPP) dispersed in poly(dimethylsiloxane) (PDMS) while the other consists of tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II) dichloride (Rudpp) dispersed in the same binder. The PtTFPP/PDMS sensor features a strong SV response that is spatially homogeneous on length scales ranging from 5 trm to several millimeters. By contrast, the Rudpp/PDMS sensor features a poor SV response. Moreover, the microscopy studies reveal that the SV response of the Rudpp sensor is spatially heterogeneous, even on length scales < 5 m. The difference in performance of the two sensor formulations is clearly due to the different solubility of the luminescent dyes in the polymer matrix. PtTFPP is compatible with the nonpolar PDMS matrix and thus disperses well in the binder, while the more polar Rudpp dye is less soluble in the PDMS and consequently microphase separation occurs in the sensor film. The results have broad implications in the formulation of oxygen sensor films for application as "pressure sensitive paints" (PSP).