In situ measurements by mid-IR laser absorption spectroscopy of C2H4/Ar pyrolysis and C2H4/O2/Ar oxidation activated by a nanosecond repetitively pulsed plasma have been conducted in a low temperature flow reactor (below 500 K) at a pressure of 60 Torr for both a continuously pulsed plasma discharge mode and a burst mode with 150 pulses. The measurements of the in situ diagnostics are validated and complemented by gas chromatography in the continuous discharge mode. A recently developed kinetic mechanism (HP-Mech) for plasma activated C2H4 oxidation is assembled. The experiments of plasma activated pyrolysis show that the formation of acetylene by direct electron impact dissociation and dissociation by excited and ionized argon collision reactions is the major fuel consumption pathway. Plasma activated C2H4 oxidation experiments show that there exist three fuel consumption pathways, 1) a plasma activated low temperature fuel oxidation pathway via RO2 chemistry; 2) a direct fragmentation pathway via collisional dissociation by electrons, ions, and electronically excited molecules; and 3) a high temperature oxidation pathway by plasma generated radicals. It is found that the plasma activated low temperature oxidation pathway is dominant and leads to a large amount of formaldehyde formation with less acetylene and methane and negligible large hydrocarbon molecules as compared to the pyrolysis experiment. The results also indicate that the latter two fuel consumption pathways are strongly dependent on O2 and Ar concentrations due to their effect on the production of atomic oxygen and excited Ar. Although the current model improves the overall prediction over USC-Mech II for plasma activated pyrolysis and oxidation, both models fail to predict quantitatively the H2O and CH4 formation. The present data provide good targets for future model development in plasma-assisted combustion.