Velocity compaction is an ion focusing principle applicable to pulsed ion source time-of-flight mass spectrometers, and is designed for achieving improved mass resolution. A quantitative view of the effect of velocity compaction focusing on a transiting iso-mass ion packet is shown. Derived equations relating the various physical dimensions of the mass spectrometer and the method of ion extraction with the time-dependent accelerating voltage required to effect velocity compaction are presented. Predicted spectra and mass resolution based on computer simulation studies are compared with actual performance in the lower mass range up to about 300 amu. The choice of initial ion temperature in the source region affects the overall mass resolution as expected, but the effect is markedly reduced by the velocity compaction focusing section. Expected mass spectra for temperatures as high as 5000 K have been obtained and indicate that unit mass separation at mass 4000 amu is possible at this extreme value.