Effects of water vapor presence in Martian atmospheric entry plasma (MAEP) were calculated based on entry data from the Viking, Pathfinder, and the Mars Exploration Rover Opportunity Landers. Calculations of the temperature and electron density across the shock front were made. We employed steady state and non-steady state kinetic models to describe the rate of dissociation of CO 2 in the Martian atmosphere as well as the production of O 2. Water vapor was included in the models since it represents 0.03% of the surface atmospheric composition. With the addition of small amounts of water vapor, we found a decrease in the dissociation of CO 2 in the Martian air as well as an increase in the production of O 2 .