The formation of ketene (H2CCO) in methane-carbon monoxide (CH4-CO) ices was investigated upon its exposure to ionizing radiation in the form of energetic electrons at 5.5 K. The radiation-induced nonthermal equilibrium processing of these ices was monitored online and in situ via infrared spectroscopy complimented with post-irradiation temperature programmed desorption studies exploiting highly sensitive reflectron time-of-flight mass spectrometry (ReTOF) coupled with single photon fragment-free photo ionization (PI) at 10.49 eV. The detection of ketene in irradiated (isotopically labeled) methane-carbon monoxide ices was confirmed via the ν2 infrared absorption band and substantiated during the warm-up phase based on sublimation profiles obtained from the ReTOF-PI spectra of the corresponding isotopic masses. The experiments conducted with the mixed isotopic ices of 12CD4-13CO provide clear evidence of the formation of at least two ketene isotopomers (D2 12C13CO and D2 13C13CO), allowing for the derivation of two competing formation pathways. We have also proposed underlying reaction mechanisms to the formation of ketene based on kinetic fitting of the temporal evolution of the ketene isotopomers. © 2014. The American Astronomical Society. All rights reserved.