[en] pH-dependent aggregation of thiol-capped CdTe quantum dots (QDs) in solutions was observed with a confocal microscope. The average size of the QD aggregates increased from 28 nm to 1.4 μm as the pH decreased from 12 to 3. The basic condition improved the dispersion of QDs while the acidic condition caused the detachment of surface ligands, leading to the aggregation of QDs. A PL lifetime of 80 ns was detected for QDs at pH from 12 to 7, while it was shortened to 57 and 34 ns at pH 5 and 3, respectively, due to the formation of surface defects