Three‐dimensional atom probe (3DAP) was employed to study nanoscale compositional heterogeneities in Co‐22 at. %Cr sputter‐deposited thin films. Compositional fluctuations were visualized on a nanometer scale through a 3D reconstruction of collected atoms. Transmission electron microscopy observations of the same specimen area of before and after 3DAP analysis made it possible to scale the analyzed volume precisely. The resulting data show that a Co‐22 at. %Cr thin film sputter deposited at elevated temperature was composed of two phases with a lamellarlike structure, one phase was ferromagnetic containing approximately 90 at. %Co, and the other was paramagnetic, containing approximately 60 at. %Co. ©1995 American Institute of Physics.