X-ray diffraction (XRD) measurements were performed on workpieces of Ni-base superalloys on bulk specimens and on powder samples of extracted precipitates. The precipitates in the bulk samples are embedded in a gamma-phase matrix and the precipitate lattice is thus in a constrained condition. The extracted precipitates have no such constrain imposed by the matrix lattice and, therefore, their lattice is in a relaxed condition. The aim of our investigations was to determine the lattice constants of various phases in two different Ni-alloys in order to estimate the lattice misfit values between the precipitates and the matrix. The two alloys are: Inconel 706 (IN706) containing three phases (gamma, gamma' and gamma") and the other is a W-rich single crystal alloy with two phases (gamma and gamma'). Differently heat treated alloys with different morphology of precipitates show slightly different diffraction patterns. The completely overlapping peaks from precipitate and matrix phases were analyzed. The particle size and the lattice strain effects on peak broadening were taken into account in the peak separation procedure. For both alloys an extraction of precipitates was analyzed as well. The results from the two samples are discussed in terms of their microstructures.