A structural and thermodynamic study of the DyNi_5-xAl_x-hydrogen system has been performed. It was found that the hexagonal crystal structure of the prototype compound DyNi₅(CaCu₅ type; space group, P6/mmm) exists up to DyNi₃Al₂. Beyond this composition and up to DyNi₂Al₃ another related hexagonal structure (YCo₃Ga₂ type; space group, P6/mmm) is stable. All alloys have been exposed to hydrogen gas at pressures up to 15 MPa and temperatures between 77 and 700 K. Under these conditions, ternary alloys having the CaCu₅ structure react with hydrogen. Their crystal structures remain the same but the hydrogen uptake increases the unit-cell volume by up to 13%. The remaining ternary alloys as well as the binary DyNi₅ do not exhibit any significant hydrogen absorption. The pressure-composition isotherms were measured. The entropy and the heat of formation have been extracted for systems that exhibit an equilibrium pressure plateau. It was found that the hydrogen capacity at ambient temperature and the equilibrium pressure decrease with the increasing content of aluminium.