Previously we proposed the concept of dual function pH and oxygen paramagnetic probes based on the incorporation of ionizable groups into the structure of persistent triarylmethyl radicals, TAMs (J. Am. Chem. Soc. 2007, 129, 7240-7241). In this paper, we synthesized an asymmetric monophosphonated TAM probe with the simplest doublet hfs pattern ideally suited for dual function electron paramagnetic resonance (EPR)-based applications. An extraordinary low line width of the synthesized deuterated derivative, p(1)TAM-D (Delta H-pp <= 50 mG, Lorentz line width, <= 20 mG) results in high sensitivity to pO(2) due to oxygen-induced line broadening (Delta LW/Delta pO(2) approximate to 0.5 mG/mmHg or approximate to 400 mG/mM); accuracy of pO(2) measurement, approximate to 1 mmHg). The presence of a phosphono group in the p(1)TAM-D structure provides pH sensitivity to its EPR spectra in the physiological range of pH from 5.9 to 8.2 with the ratio of signal intensities of protonated and deprotonated states being a reliable pH marker (accuracy of pH measurements, +/- 0.05). The independent character of pH and [O-2] effects on the EPR spectra of p(1)TAM-D provides dual functionality to this probe. The L-band EPR studies performed in breast tumor-bearing mice show a significant difference in extracellular pH and pO(2) between tumor and normal mammary gland tissues, as well as the effect of animal breathing with 100% O-2 on tissue oxygenation. The developed dual function phosphonated p(1)TAM-D probe provides a unique tool for in vivo concurrent tissue oxygen and pH monitoring.