Hydrogen-carrying compounds have accrued interest as an alternative to traditional fossil fuels due to their function as zero-emission fuels. As such, there is interest in investigating hydrogen-carrying compounds to improve understanding of the fuels' characteristics for use in high-pressure systems. In the current study, the oxidation of ammonia/natural gas/hydrogen mixtures was carried out to study CO formation as well as ignition delay times behind reflected shock waves in order to refine chemical kinetic models. Experiments were carried out in a shock tube facility by utilizing chemiluminescence to obtain OH* emission and laser absorption spectroscopy to obtain CO profiles over a temperature range between 1200 K to 1800 K with an average pressure of 2.2 atm. Experimental mixtures included neat and combination natural gas/hydrogen with ammonia addition, with all mixtures except one having an equivalence ratio of 1. Results were compared with the GRI 3.0 mechanism and the newly developed UCF 2022 mechanism utilizing CHEMKIN-Pro software. In general, models were able to capture the trend in autoignition delay times and CO time histories for natural gas and ammonia mixtures. For ammonia-hydrogen mixtures, GRI 3.0 failed to predict ignition delay times, whereas the UCF 2022 mechanism was able to capture the IDTs within uncertainty limits. A sensitivity analysis was conducted to understand the important reactions at the experimental conditions. Finally, a reaction pathway analysis was carried out to understand ammonia decomposition pathways in the presence of hydrogen and natural gas.