In the antiferromagnetic ground state of α-NiMoO 4 the magnetic moments within each sublattice are collinearly aligned in the ac-plane, but with a slightly different orientation for both sublattices. The magnetic phase diagrams have been determined for three different directions of the applied field, parallel to [010], [101] and within the ac-plane perpendicular to [101]. The external fields, applied within the ac-plane, induce two successive phase transitions, into a spin-flop phase first and into the forced ferromagnetic state second. In the spin-flop phase the magnetic moments are aligned in the [010] direction. A field parallel to [010] induces only one phase transition into the forced ferromagnetic state.