A new series of ionic liquids based on the difluorophosphate anion (PO(2)F(2)(-)) has been synthesized by metathesis from the corresponding chloride. Incorporation of the PO(2)F(2)(-) anion with an N-heterocyclic cation (1-ethyl-3-methylimidazolium (EMIm(+)), 1-butyl-3-methylimidazolium, 1-butyl-1-methylpyrrolidinium, or 1-butyl-1-methylpiperidinium (BMPip(+))) produces ionic liquids with low melting points, including some room-temperature ionic liquids. The vibrational frequencies of PO(2)F(2)(-) in the obtained ionic liquids were assigned with the aid of quantum mechanical calculations. The ionic conductivities of the ionic liquids are comparable to those of the corresponding tetrafluoroborate-based ionic liquids (e.g., 12 mS cm(-1) for 1-ethyl-3-methylimidazolium difluorophosphate, [EMIm][PO(2)F(2)]). The stability of ionic liquids against hydrolysis is significantly improved by replacing the hexafluorophosphate anion with the difluorophosphate anion, which suppresses the subsequent liberation of hydrogen fluoride. According to the solvatochromic method, the donor ability of ionic liquids based on PO(2)F(2)(-) is stronger than those of ionic liquids based on BF(4)(-), PF(6)(-), and N(SO(2)CF(3))(2)(-), due to the large negative charge on the oxygen atoms in PO(2)F(2)(-). The electrochemical window of [EMIm][PO(2)F(2)] is around 4.2 V, and the diffusion coefficients of ferrocene in [EMIm][PO(2)F(2)] are 2.83 x 10(-7) cm(2) s(-1) by cyclic voltammetry and 3.03 x 10(-7) cm(2) s(-1) by chronoamperometry, both of which are comparable to those in 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIm][BF(4)]). The X-ray structure of [BMPip][PO(2)F(2)] contains two cations and two anions in the asymmetric unit, where one of the two cations is disordered into two positions (chair configurations) even at -173 degrees C. The oxygen atoms in PO(2)F(2)(-) have closer contacts to the hydrogen atoms in BMPip(+) than the fluorine atoms, indicative of their stronger basicity.