The magnetism of antiferromagnetic FexPt1-x thin films (x = 0.27 and 0.30) epitaxially grown onto MgO (110) and a-axis sapphire (alpha -Al2O3) substrates has been studied by elastic neutron and x-ray scattering. Bulk chemically ordered FePt3 exhibits an antiferromagnetic spin structure with a wave vector Q(1) =2 pi /a(1/2 1/2 0) below T(N1)similar to 160 K. For slightly Fe-rich alloys (x greater than or similar to0.26) a spin-reorientation transition ti, a second antiferromagnetic phase with a wave vector Q(2) = 2 pi /a(1/2 00) occurs below T(N2)similar to 100 K at the expense of Q(1). For increased Fe content (x similar to0.30) the Q(1) phase is strongly suppressed with a dominant Q(2) phase. For (111)-oriented films grown on a-axis sapphire the spin structure is the same as that found in the bulk. The a = 0.27 film exhibits transitions at T-N1 and T-N2. The film with x = 0.30 exhibits an almost completely suppressed Q(1) phase and a dominant Q(2) phase with an enhanced ordering temperature of T(N2)similar to 140 K. In contrast FePt3 (110) films grown onto MgO (110) exhibit only the el phase for both compositions x = 0.27 and x = 0.30 with the onset of the Q(2) phase suppressed. The distinct behavior of the films grown onto MgO from those grown onto a-axis sapphire and bulk FePt3 may be explained by higher strain and defect densities incorporated in the films grown onto MgO.