IR difference spectra obtained before and after 200 keV H⁺ radiolysis of (a) C₂H₂:H₂O mixed ice along with (b) C₂H₂:H₂O and (c) C₂H₂:H₂¹⁸O layered ices for the same H⁺ fluence of 6.0 × 10¹⁴ protons cm⁻² (i.e., ~ 16.4 eV 16u-molecule⁻¹) at 17 K. The black arrows followed by dashed lines indicate the absorption peaks of regular COMs with ¹⁶O-atoms, and red arrows indicate the absorption peaks of COMs with ¹⁸O-atoms. IR spectra are offset for clarity.