We have studied the magnetic properties of CoNi binary alloy films with various atomic compositions using the soft x-ray magnetic circular dichroism technique. The alloy films were deposited on a single Cu(100) crystal in situ using our well established epitaxial growth technique to achieve a layer-by-layer growth and a metastable face-centered-cubic structure, with all the films exhibiting an in-plane magnetic anisotropy. The high density, circularly polarized x-ray beam was supplied by the elliptically polarizing undulator at the Stanford Synchrotron Radiation Laboratory. Utilizing the element-specific ability and nanostructure magnetization sensitivity of this technique, we were able to perform the absorption measurements at L-2 and L-3 edges of Co and Ni atoms and we observed large dichroism signals. The extraction of spin moment and orbital moment for varying elemental stoichiometry using magneo-optical sum rules is discussed.