The crystal structure of a new monoclinic form of potassium dihydrogen phosphate, (\mathrm{KH}{2} \mathrm{PO}{4}), has been determined by single crystal X-ray diffraction. Crystals are monoclinic, space group (P 2 / c) with (a=7.4399(7), b=7.2634(9), c=9.3629(13) \AA, \beta=) (127.696(8)^{\circ}, V=400.35(9) \AA^{3}, d_{m}=2.25, d_{c}=2.257 \mathrm{Mg} \mathrm{m}^{-3}) for (Z=4, \mu=17.9 \mathrm{~cm}^{-1}, F(000)=272, T=293 \mathrm{~K}). The structure was refined to (R=0.022) and (R_{\mathrm{w}}=0.038) for 740 reflections with (I \geq 3 \sigma(I)). The crystal structure consists of a compact assembly of potassium and phosphate ions, arranged in columns along the (b)-axis, bound by (\mathrm{K} \cdots \mathrm{O}) ionic bonds and (\mathrm{O}-\mathrm{H} \cdots \mathrm{O}) hydrogen bonds in a three-dimensional network as in all other forms of (\mathrm{KH}{2} \mathrm{PO}{4}) and (\mathrm{KD}{2} \mathrm{PO}{4}). The two crystallographically nonequivalent phosphate groups appear to exist as an orthophosphacidium ion, (\left[\mathrm{H}{4} \mathrm{PO}{4}\right]^{+})and a (\mathrm{PO}{4}^{-3}) ion rather than as the expected (\mathrm{H}{2} \mathrm{PO}{4}) ions. This study provides the first example of crystal structure determination of the (\left[\mathrm{P}(\mathrm{OH}){4}\right]^{+})ion. 1995 Academic Press, Inc.