We propose to study the local structure in two series of promising thermoelectric materials: Ti1-xZrxNiSn and TiNi1+ySn. Diffraction experiments on Ti1-xZrxNiSn reveal multiple half-Heusler phases, while the TiNi1+ySn samples show a segregation of the excess Ni (y) in regions with composition TiNi2Sn. Pair distribution function (PDF) analysis will enable us to determine the bond distances in these series, and yield information on the local structure, which is relevant to understanding the low latice thermal conductivities in these materials.