Phase-pure NaCN(3)H(4) and KCN(3)H(4) were synthesized from molecular guanidine and elemental metal in liquid ammonia at room temperature and elevated pressure close to 10 atm. The crystal structures were determined at 100 K using single-crystal X-ray diffraction. Both compounds crystallize in the monoclinic system (P2(1)/c, No. 14) but are far from being isotypical. NaCN(3)H(4) (a = 7.9496(12) Å, b = 5.0328(8) Å, c = 9.3591(15) Å, β = 110.797(3)°, Z = 4) contains a tetrahedrally N-coordinated sodium cation while KCN(3)H(4) (a = 7.1200(9) Å, b = 6.9385(9) Å, c = 30.404(4) Å, β = 94.626(2)°, Z = 16) features a very large c axis and a rather complicated packing of irregularly N-coordinated potassium cations. In the crystal structures, the guanidinate anions resemble the motif known from RbCN(3)H(4), that is, with one elongated C-((amino))N single bond and two shorter C-((imino))N bonds (bond order = 1.5) although the orientation of one N-H bond differs in the guanidinate anion of NaCN(3)H(4). Both crystal structures and infrared spectroscopy evidence the presence of hydrogen-bridging bonds, and the vibrational properties were analyzed by ab initio phonon calculations.