A system for testing and adjusting threshold voltages in flash EEPROMs is disclosed. The system includes a memory cell array (5) comprising a plurality of cell columns. Each cell column includes a plurality of memory cells (10). Each memory cell (10) has a control gate terminal (14), a drain terminal (12) and a source terminal 11. A control system comprising a wordline decoder (16), a column decoder (19) and a microprocessor (21) applies selected voltages to the respective terminals of the memory cells (10), and selects one of the plurality of cell columns for compaction verification. A detector (30) determines whether any one of the memory cells (10) of the selected cell column has a threshold voltage below a predetermined positive voltage, and supplies an output signal to the control system. The control system increases respective threshold voltages of the memory cells (10) of the selected cell column in response to the output signal of the detector (30).