Absolute partial cross sections from threshold to 1000 eV are reported for electron-impact ionization of Ar, H\sb2, N\sb2, O\sb2, and CO\sb2. Data are presented for the production of Ar\spn+ (n=1−4) from Ar; the production of H\sbsp2+ and H\sp+ from H\sb2; the production of N\sbsp2+, N\sp++N\sbsp22+, and N\sp2+ from N\sb2; the production of O\sbsp2+, O\sp++O\sbsp22+, and O\sp2+ from O\sb2; and the production of CO\sbsp2+, CO\sp+, CO\sbsp22+, O\sp+, C\sp+, O\sp2+, and C\sp2+ from CO\sb2. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output provides a clear demonstration of the complete collection of energetic fragment ions from dissociative ionization. For singly charged parent ions, the overall uncertainty in the absolute value of the cross sections reported here is $\pm$3.5% and is marginally higher for multiple and dissociative ionization. Previous cross section measurements are compared to the present results. Additionally, combining the flight time and detection location of fragment ions yields some qualitative information about their energy distribution.