Abstract: Six chemicals, H₂, H₂O, CO₂, SF₆, CF₃Br and Ni(CO)₄, are considered as ionospheric modification materials. Each of these chemicals reacts in the F region to produce localized plasma depletions and form large scale electron holes. In the active ionospheric modification experiments, the criteria for selecting the released chemicals include the cost and effectiveness. The effectiveness of released chemicals depends on the amount which goes into the vapor state. In this paper, the thermodynamics governing the vapor fraction of the released chemicals were investigated. Additionally, the size and magnitude of the electron depletions were calculated with finite element simulation method. The results show that H₂O has the lowest vapor yield of about 19% from a heated, pressurized tank, and over 60% of the other five chemicals should be vented in gaseous form. To reduce the cost, the chemicals with small density, such as H₂ or CO₂ may be chosen. Based on estimating the electron density reduction, the chemicals which have slow diffusion speed and large chemical reaction rates, such as SF₆ or Ni(CO)₄, may be selected. These chemicals can produce a larger, deeper and more durable electron hole.