Transient Langmuir probe measurements of the plasma density, electron temperature, and plasma potential during “slow quenching’’ of a Cs–Ba tacitron at two different discharge conditions were made. Following the application of a negative voltage pulse on the grid, the grid aperture sheaths initially grew then shrank temporarily, due to an increase in the ion flux from the grid-collector region, then grew again until the discharge eventually extinguished. Discharge extinguishing occurred when the effective ionization cross section became decoupled from the continually rising electron temperature; hence, the discharge could not be maintained owing to the decrease in the ionization rate. This inability to maintain the discharge is considered the mechanism for extinguishing by slow quenching. ©1997 American Institute of Physics.