The magnetic properties of the metamagnet Eu3O4 are analyzed using molecular-field theory. For the antiferromagnetic ground state, it is assumed that linear, ferromagnetic chains of spins (Eu2+) are polarized along the c axis of the orthorhombic unit cell, with neighboring chains antiparallel. The weakly magnetic ions Eu3+ provide an apparently inert magnetic background. Exchange constants are deduced from the paramagnetic Curie temperature θp=5°K and the critical field Hc=2.4 kOe. The constants for first- and second-neighbor Eu2+ interactions are J1k~0.3°K and J2k~-0.02°K, respectively. An anisotropy field of 7 kOe is derived from the transverse magnetic susceptibility at low temperatures. This value is also obtained from a magnetic dipolar sum. Theoretical estimates of the ordering temperature give Tc~5°K, as observed.