In this work sublimation of porous water ice in vacuum is investigated. The experiments were performed at low temperature gradient of the order 10 K m⁻¹. It was found, that the surface of porous granular ice can recede much slower than of compact ice. When the porosity is 0.75, as relevant for the nuclei of comets, then the rate of recession can be about 60% lower than in the case of compact ice. In addition, preliminary experiments performed at moderate and high temperature gradient are described.