Optical chemical sensors -with few exceptions -rely on the use of smart probes and materials that respond to the species of interest by change in their optical properties, often in luminescence. [1][2][3][4][5][6][7][8][9][10] They have the specific option of optical multiplexing. In other words, sensors can be designed so that they give a multitude of spectral and time-dependent information that, in turn, enables sensing of several parameters simultaneously if the signals can be separated and attributed in an unambiguous way. We and others [11][12][13][14][15][16] have previously designed dual sensors, for example for oxygen and temperature or oxygen and pH, and related multiplex approaches (with one probe responding to more than one parameter) have been reported recently. [17][18][19] Our interest in sensors for pH, temperature (T), and O 2 (in gaseous or dissolved forms) results from the fact that these are the parameters probably determined most often in chemistry, biology, environmental sciences, numerous industrial areas, and also in more specific areas such as clinical chemistry or marine sciences. Electrochemical devices that measure these parameters do exist, are widely distributed, and perform fairly well. Optical single sensors for these species have also existed for many years, [1] are less common, and have specific merits because information is gathered via photons rather than electrons, often in combination with fiber-optic light guides. This can substantially reduce the risk of explosions in chemical plants, enables sensing at patients with heart pacemakers and in strong electromagnetic fields, and -in case of fiber optics -paves the way to sensing over large distances.At first glance, it would appear that triple sensing can be achieved by simply using the three best working indicator probes (one each for pH, T, and O 2 ) with highly different optical spectra and to incorporate them into an appropriate polymer matrix, which is then exposed to the sample to be analyzed. A closer look into the situation reveals that the solution is not as simple for several reasons. Notably, the spectral overlap of practically all indicator probes results in substantial spectral crosstalk. In fact, most probes have bands that extend over more than 150 nm in width, so that the spectral range that can be exploited in practice (400-750 nm) is almost fully covered by two indicator dyes, not considering the fact that pH probes exist in two forms depending on pH. A second and quite serious limitation results from the effect of fluorescence resonance energy transfer (FRET) whenever indicator probes with overlapping bands are applied in high concentrations so that the critical distance for FRET to occur (typically 5-7 nm) is reached; this can result in heavy crosstalk of signals. Third, pH-dependent color changes are likely to lead to inner filter effects, which are disadvantageous if luminescence intensity (rather than lifetime) is measured. Finally, each of the three probes requires an optimized polymer matrix that not onl...