New materials are described that lead to sensors capable of simultaneous sensing of pH and oxygen via a single-fiber optic sensor. They make use of a pH probe based on carboxyfluorescein, and of a ruthenium(II) complex acting as a probe for dissolved oxygen. The selectivity of the probes was considerably improved by incorporating them into two kinds of microparticles, each of specific permeation selectivity. The pH probe was immobilized on particles made from proton permeable amino-modified poly(hydroxyethyl methacrylate), while the oxygen probe was physically immobilized in beads made from an organically modified sol-gel. Both kinds of beads were then dispersed into a hydrogel matrix and placed at the distal end of an optical fiber waveguide for optical interrogation. A phase-modulated blue-green LED serves as the light source for exciting luminescence whose average decay times or phase shifts serve as the analytical information. Data are evaluated by a modified dual luminophore referencing (m-DLR) method which relates the phase shift (as measured at two different frequencies) to pH and to oxygen partial pressure. The dually sensing material performs best if the sensing matrix is very homogeneous and if the microbeads have a diameter of < 3 mu m.