We determine the carbon selectivity for single-step conversion of natural gas to an acetic acid/ methanol liquid mixture by a new homogeneous catalytic process. The amount of CO2 produced as a byproduct needs to be less than 25% of the total product for the process to be of interest for industrial development. Using a combination of nitrogen blank runs and C-13 labeling along with gas chromatography and mass spectrometry, we show that (1) a lower pressure of methane gives better conversion, (2) most of the CO2 produced comes from decomposition of the trifluoroacetic acid solvent, (3) the reaction is quicker than previously thought, and (4) the product selectivity to liquids is typically around 75%.