Whole rock subsurface samples from the Cherry Canyon Formation, Delaware Basin, were used to document the presence of a contaminant gas affecting the isotopic composition of acidified carbonate. The whole rock material, when reacted in purified phosphoric acid using the McCrea (1950) method, produces a contaminant mass-46 gas (nitrogen dioxide, NO 2 ). This mass-46 gas contributes to the 18 O/ 16 O ratio (mass 46/44), artificially enriching the delta 18 O value and masking the true isotope signature of the CO 2 derived from the carbonate minerals. Acidified whole rock samples containing low carbonate (< 5%) tend to yield more positive delta 18 O (PDB) values, some greater than +30 per thousand . Conversely, samples with higher carbonate (> 5%) have delta 18 O values ranging from +5 per thousand to -8 per thousand , hence diluting the effects of the contaminant gas on the mass 46/44 ratio. After purifying the CO 2 produced by the McCrea (1950) method with a reduction furnace, all of the delta 18 O values fall in the range of 0 per thousand to - 12 per thousand regardless of carbonate content. The delta 16 C (PDB) values remain unchanged after reduction furnace purification, indicating no effect on mass 45/44 ratio. The inferred source of the contaminant NO 2 gas is from inorganic ammonium adsorbed to interlayer sites of alumina silicates such as illite and mica. This source is supported by the detection of ammonium and clays in the whole rock material and the lack of correlation between percent organic carbon and delta 18 O values influenced by NO 2 . To eliminate the effect of the contaminant gas on delta 18 O values, we describe a simple procedure that removes the contaminant NO 2 gas, yielding the true isotopic signal from the carbonate minerals in whole rock material.