This paper presents our recent results on synthesis and properties of high-pressure cubic zirconium(IV)- and hafnium(IV) nitrides having Th 3 P 4 -type structure, c-M 3 N 4 (M=Zr or Hf). c-M 3 N 4 were first synthesized in microscopic amounts in a laser-heated diamond anvil cell (LH-DAC) at pressures above 15GPa and temperatures above 2500K. Further, macroscopic amounts of c-Hf 3 N 4 and of a new oxygen-bearing zirconium nitride, c-Zr 2.86 (N 0.88 O 0.12 ) 4 , were synthesized at 12GPa and about 1900K using a multi-anvil apparatus. The lattice parameter of c-Hf 3 N 4 was determined to be a 0 =670.2(1)pm. The a 0 of c-Zr 2.86 (N 0.88 O 0.12 ) 4 of 675.49(1)pm was found to be slightly larger than that of c-Zr 3 N 4 (674.0(6)pm). The bulk moduli, B 0 , of 219GPa for c-Zr 2.86 (N 0.88 O 0.12 ) 4 and of 227GPa for c-Hf 3 N 4 were obtained from compression measurements. Combining the compression and indentation data we determined for c-Zr 2.86 (N 0.88 O 0.12 ) 4 its isotropic shear modulus G 0 =96GPa. Finally, Vickers microhardness, indentation fracture toughness and thermal expansion of c-Zr 2.86 (N 0.88 O 0.12 ) 4 were measured.