•Mixtures of TiNiSn and Ni nanoparticles were consolidated by spark plasma sintering.•The initial preparation method of TiNiSn and SPS pressing conditions were varied.•The structure, microstructure, and thermoelectric properties differ significantly.•We suggest different scattering behavior of the various secondary phases.•Samples with around 5% of TiNi2Sn show the lowest lattice thermal conductivity.The electronic and thermal properties of thermoelectric materials are highly dependent on their microstructure and therefore on the preparation conditions, including the initial synthesis and, if applicable, densification of the obtained powders. Introduction of secondary phases on the nano- and/or microscale is widely used to improve the thermoelectric figure of merit by reduction of the thermal conductivity. In order to understand the effect of the preparation technique on structure and properties, we have studied the thermoelectric properties of the well-known half-Heusler TiNiSn with addition of a small amount of nickel nanoparticles. The different parameters are the initial synthesis (levitation melting and microwave heating), the amount of nickel nanoparticles added and the exact pressing profile using spark plasma sintering. The resulting materials have been characterized by synchrotron X-ray diffraction and microprobe measurements and their thermoelectric properties are investigated. We found the lowest (lattice) thermal conductivity in samples with full-Heusler TiNi2Sn and Ni3Sn4 as secondary phases.