Cross sections for the production of positive ions by electron impact on NH3 and CS2 have been measured using a crossed electron beam-molecular beam collision geometry. The present results on NH3 are considerably different from previous measurements which disagree among themselves. Except for the formation of CS2+, cross sections for other fragment ions resulting from CS2 have not been measured in the past. Threshold energies for the appearance of various positive ions are also reported. It is estimated that the present results are accurate to within 15%. ¿ American Geophysical Union 1991