O)*

The multiferroic behavior of any material sensitively depends on the microscopic interactions between the spins. We evaluate the magnetic interactions in the multiferroic erythrodsiderite (NH₄)₂FeCl₅ (H₂O) by comparing inelastic neutron scattering spectra of a single crystal sample with a simple Heisenberg model containing five exchange interactions and an easy-plane anisotropy. The cycloidal spin state in every bc plane is produced by two competing exchange interactions. Using the observed wavevector of this cycloidal spin state is used as a constraint, excellent agreement is found between the observed and predicted spectra. The resulting exchange and anisotropy parameters are compared with the predictions of first-principle calculations.