This collection contains the experimental data for the manuscript titled: "Phase transition of CO ice: driving the molecular complexity on interstellar dust grains". This collection contains mid-infrared spectral data at a resolution of 1 cm-1. The temperature corresponding to each spectrum is marked in the "temperature" column or in the file name. The background spectrum of each experiment is taken to be the one measured right before the CO/CO2 deposition. There are three folders corresponding to three sets of experiments: 1. CO warm-up. RAIRS spectra of CO ice of different thicknesses during warm-up. The file is named by the thickness. The CO ices were deposited onto np-ASW at 6 K and warmed up at 0.2 K/minute. The temperature corresponding to each spectrum is marked in the "temperature" column. 2. CO:CO2 mixture warm-up. RAIRS spectra of the mixture of 9 ML of CO and 1 ML of CO2 during warm-up. The CO:CO2 ice mixture was deposited onto np-ASW at 6 K and warmed up at 0.2 K/minute. The temperature corresponding to each spectrum is marked in the "temperature" column. 3. CO:CO2 mixture isotherm. RAIRS spectra of the mixture of 9 ML of CO and 1 ML of CO2 deposited at 6 K and then flash heated to a target temperature to do isotherm experiments. The target temperatures are shown in the file name.