Evolution of the surfaces of cometary nuclei is determined by the sublimation of ice. The rate of sublimation is commonly calculated using the simple Hertz–Knudsen formula, which should be corrected by a temperature dependent sublimation coefficient αs (Kossacki et al., 1999; Gundlach et al., 2011; Kossacki and Leliwa–Kopystynski, 2014). In this work influence of C3H6O (acetone), and CH3OH (methanol) present in cometary ice on the sublimation coefficient is investigated. We have found, that the mass fraction of admixtures ≤ 2% in water ice is sufficient for an increase of αs by a factor about 5, depending on the temperature. The sublimation coefficient is sensitive to very small concentrations of admixtures. When the admixture is acetone and the mass fraction is αs(215 K) ∼ 0.46 instead of ∼ 0.18 for pure water ice. When is αs(215 K) ∼ 0.74, and when is αs(215 K) ∼ 0.78. The presence of 0.01 mass fraction of methanol has similar influence on the sublimation coefficient as the presence of 0.01 mass fraction of acetone. At temperatures T > 235 K for all investigated compositions of ice αs ∼ 0.15.