An asymmetric derivative of the triarylmethyl radical, TAM-H, containing one aldehyde and two carboxyl groups, was synthesized. The electron paramagnetic resonance (EPR) spectrum of TAM-H is characterized by a doublet of narrow lines with a linewidth of 105 mG under anoxic conditions and hyperfine interaction constant of 245 mG. The partial overlap of the components of the doublet results in enhanced sensitivity of the spectral amplitudes ratio to oxygen compared with oxygen-induced linewidth broadening of a single line. Application of the TAM-H probe allows for EPR measurements in an extended range of oxygen pressures from atmospheric to 1 mm Hg, whereas the EPR spectrum linewidth of the popular TAM-based oxygen sensor Oxo63 is practically insensitive to oxygen partial pressures below 20 mm Hg. Enhanced sensitivity of the TAM-H probe relative to Oxo63 was demonstrated in the detection of oxygen consumption by Met-1 cancer cells. The TAM-H probe allowed prolonged measurements of oxygen depletion during the hypoxia stage and down to true anoxia (<or=1.5 mm Hg).