Microwell arrays have been fabricated on the distal face of coherent fiber-optic bundles. A typical microwell array comprises approximately 3,000 individual optical fibers that were etched chemically. Individual microwells were 1 to 14-microm deep with approximately 22-microm widths and were filled partially with a chemical sensing (polymer + dye) layer to produce a microwell array sensor (MWAS). MWASs were fabricated using a technically expedient, photoinitiated polymerization reaction whereby a approximately 2 to 10-microm thick pH-sensitive or O2-sensitive sensing layer was immobilized inside each microwell. The pH-sensing layer comprised fluorescein isothiocyanate-dextran conjugate immobilized in a photopolymerizable poly(vinyl alcohol) membrane. The O2-sensing layer comprised a ruthenium metal complex entrapped in a gas-permeable photopolymerizable siloxane membrane. pH and PO2 were quantitated by acquiring luminescence images using an epifluorescence microscope/charge-coupled device imaging system. The pH-sensitive MWAS displayed a pKa of approximately 6.4 and a response time of approximately 2.5 s. The O2-sensitive MWAS behaved according to a nonlinear Stern-Volmer model with a maximum I0/I of approximately 4 and a response time of approximately 2.5 s. MWASs are advantageous in that suitably sized samples such as single biological cells can be co-localized with the sensing matrix in individual microwells.