Absolute partial and total cross sections for electron-impact ionization of CH3F, CH3Cl, CH3Br, and CH3I are reported for electron energies from threshold to 1000 eV. The product ions are mass analyzed using a time-of-flight mass spectrometer and detected with a position-sensitive detector whose output demonstrates that all product ion species are collected with equal efficiency irrespective of their initial kinetic energies. Data are presented for the production of CHnF+, CHnCl+, CHnBr+, CHnI+, CHn+, F+, Cl+, Br+, I+, and H+, where n=0-3. Data are also reported for production of H-2(+), H-3(+), and doubly charged species at 100 eV. The total cross section for each target is obtained as the sum of the partial cross sections. The overall uncertainty in the absolute cross sections for most of the singly charged ions is +/-6%. It is observed that, although the Deutsch-Mark and the binary-encounter-Bethe theoretical formalisms are able to predict some features of the measured total cross sections, neither is consistently accurate. The simple ab initio method of Vallance can apparently predict the value of the cross section maxima as well as the other more sophisticated approaches.