In order to better understand the stability of water on Mars, and factors likely to affect that stability, we have measured the evaporation rate of water in a CO2 atmosphere at 7 mbar and ~0¿C in a large environmental chamber that minimizes fluctuation in conditions. The average evaporation rate we measured was 1.01 ¿ 0.19 mm/h. This includes correction for water vapor build-up in the chamber, but not the effect of Mars' lower gravity. When corrections for this are applied, our predicted evaporation rate for water on Mars at 0¿C and 7 mbar is 0.73 ¿ 0.14 mm/h. This is in very good agreement with the value calculated by assuming that evaporation depends on diffusion and buoyancy. The consistency of these results suggests that -- at least for these conditions -- our approaches to the calculations and the laboratory simulations are reasonable.