A single K2HPO4 droplet with size of similar to 50 mu m on a Teflon substrate was forced to enter into the supersaturated state by decreasing the relative humidity (RH), allowing accurate control over the concentration of the solute within a droplet of a nanogram. The K2HPO4 solutions from dilute (0.1-1.0 mol.L-1 bulk) to concentrated state (a droplet from RH 98.2% to 25.1%) were studied through micro-Raman spectroscopy in the spectral region of about 200-4000 cm(-1). The area ratio between the water stretching band to the sum of the nu(1)-PO3, nu(2)-POH, and nu(4)-PO3 bands of the HPO42- at various RHs was used to describe the dehydration behavior of a microsized single K2HPO4 droplet in dehumidifying process. The peak position of the nu(1)-PO3 band for the 1 mol.L-1 bulk solution appeared at 991 cm(-1) and moved to 986 cm(-1) at 98.2% RH, to 978 cm(-1) at 70.2% RH, and then to 964 cm(-1) at 30.0% RH for a droplet, accompanying an increase of the full width at half-height (fwhh) of this peak from 16.3 to 17.2, 22.2, and then to 24.2 cm(-1), indicating transition of the HPO42- anions from monomers to dimers/trimers/oligomers and then to polyanions with chain structures in the K2HPO4 solutions. After 25.1% RH, the solid was proved to be K2HPO4 center dot 3H(2)O according to the Raman spectral features. Furthermore, the O-H stretching envelope of a K2HPO4 roplet showed that the intensity ratios of the strong hydrogen bonding component (3255 cm(-1)) to the weak one (3417 cm(-1)) and the cage-like water (2925 cm(-1)) to the weak one (3417 cm(-1)) were sensitive to the HPO42- association structures, which can be used to understand the effects of dimers/trimers/oligomers and chain structures of the HPO42- associations on the hydrogen bonding of water molecules.