Abstract: An improved pulsed-beam time-of-flight mass spectrometer is described whereby the velocities of a plurality of iso-mass ions are equalized (velocity compaction) by subjecting a transiting ion bunch, partially separated into iso-mass ion packets, to a time-dependent and monotonically time-varying acceleration force field. Concurrently, space compaction or space focussing is achieved through a speeding up of the retarded ions (relative to the advanced ions) in a given iso-mass ion packet. The wave-form of the ion accelerating field may be of an exponential-limiting-like form in time and depends on the various physical and voltage parameters associated with the ion source, accelerating grids and ion drift distances. When the acceleration force field is properly contoured in both space and time velocity compaction and space compaction simultaneously are achieved for a wide range of iso-mass ion packets and the mass resolution for heavier mass ions is particularly improved.