Six chemicals, H2, H2O, CO2, SF6, CF3Br, and Ni(CO)4, are considered as ionospheric modification agents. Each of these species reacts in the F region to produce localized plasma depletions. The first three interact with O+ and yield polyatomic ions which dissociatively recombine with electrons to give neutrals. The last three dissociatively attach electrons to produce heavy negative ions which become mutually neutralized by reactions with O+. The effectiveness of these chemicals depends on the amount which goes into the vapor state upon release. Thermodynamic calculations show that H2O has the lowest vapor yield of about 20% from a heated, pressurized tank. Over 60% of the other substances should be vented in gaseous form. Based on estimates of plasma density reduction and airglow stimulation, nickel carbonyl is the most efficient of the six species for modifying the nighttime ionosphere. During the daytime, CF3Br and SF6 provide the largest depletions. ¿American Geophysical Union 1987