Measurements of the vapor pressure of H 2 O and CO 2 at temperatures relevant to the middle atmosphere of Earth and Mars are rare but important in order to describe cloud formation and ice particle growth processes. In this contribution we present a novel technique for measuring the vapor pressure of condensable gases by analyzing the depositional growth rates on free nanoparticles at high supersaturation. The method is applied to measure the vapor pressure of CO 2 between 75K and 85K. By comparison with previous measurements and parameterizations we are able to show the excellent functionality of the method. In addition, the method is used to measure the vapor pressure over H 2 O ice between 135K and 160K. We show that the vapor pressure of so called stacking disordered ice I sd deposited at temperatures below 160K is significantly higher compared to hexagonal ice I h. The consequences for ice cloud formation in the atmosphere of Earth and Mars will be discussed.