X-ray diffraction based strain measurements have been carried out during the deposition and subsequent relaxation of electroless (autocatalytic) polycrystalline copper films. Thin polymer substrates were mounted on the surface of an electrolyte-filled plating cell, and the X-rays traversed the substrate to scatter of the growing Cu layer. The plating cell was rotated back and forth by up to 70° in order to find the strain of Cu crystallites within and perpendicular to the plane of the film (sin2ψ method). Three types of plating solutions were investigated. A Ni-free solution C leads to compressive strain during steady-state film growth, followed by an exponential relaxation of the film to a residual tensile strain. Electrolytes A and B contain Ni ions, and the resulting Cu(Ni) films have nearly constant strain with small counteracting strain variations during and after film growth. Cyanide-stabilized solution A yields films with a slight compressive strain, while solution B, stabilized by an aromatic nitrogen compound, yields films with tensile strain. Different and reproducible evolution patterns observed for these three electrolyte types establishes in situ X-ray diffraction strain monitoring as a method to evaluate chemical formulations for electroless deposition.