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The partial oxidation of natural gas in HCCI engines in terms of a polygeneration process could be a promising technology to flexibly produce useful chemicals, heat, and work, depending on demand. Because natural gas is relatively inert, high intake temperatures or compression ratios are required to initiate its conversion which can result in a lower lifetime of the engine. Alternatively, small amounts of more reactive species such as ozone can be added to the initial mixture to provide radicals at more moderate conditions and initiate the main ignition. In this study, plug-flow reactor experiments are performed to assess the influence of ozone on the fuel conversion and product formation during the partial oxidation of methane and natural gas. Experiments are performed in the temperature range of 373 K to 973 K at 4 bar and equivalence ratios of 2. Molecular-beam mass spectrometry coupled with electron ionization is used as analytical technique to detect the mixture composition at the outlet of the reactor. The results show that even very small amounts of ozone can help to shift the conversion onset to much lower temperatures and increase the yields of useful chemicals. The data can further be used to improve reaction mechanisms describing the conversion of hydrocarbons in the presence of ozone. A literature ozone reaction mechanism has been implemented in two recent hydrocarbon mechanisms and the results of simulations using these mechanisms are compared to the experimental data with respect to the low-temperature intermediates which influence ignition. Predictions differ substantially from the experimental results identifying starting points for further investigations. The speciation data provided in this work contribute to extending the reaction kinetics of ozone assisted fuel conversion to higher pressure. [ABSTRACT FROM AUTHOR]