Hf 5CoAs 3 was prepared by arc-melting of HfAs, Hf and Co. This new arsenide crystallizes in the orthorhombic space group Pnma with cell dimensions of a=17.327(5)Å, b=3.685(4)Å, c=10.370(4)Å, V=662(1)Å 3. Its crystal structure is isostructural to the structure of Hf 5CoP 3. The Co and As atoms both are located in trigonal prismatic voids of the Hf substructure. The Hf atoms form double channels, each of which is filled with an infinite -Co-As- zigzag chain. These double channels are interconnected via bonding Hf–Hf interactions to form the three-dimensional Hf substructure. Since adding more Co occurs with significantly enlarged cell dimensions, it is concluded that the additional Co atoms are included in the structure of Hf 5CoAs 3, as Ni is in case of Hf 5Ni 1+ x P 3. The densities of states obtained by Extended Hückel calculations and the Pauli paramagnetism experimentally obtained point to metallic behavior of Hf 5CoAs 3.