Large single crystals of ZrAs x Se y (x>y, x +yr 2, PbFCl type of structure, space group P4/nmm) were grown by Chemical Transport. Structural details were studied by single crystal neutron diffraction techniques at various temperatures. One single crystal specimen with chemical composition ZrAs 1.595(3) Se 0.393(1) was studied at ambient temperature (R1 ¼ 5.10 %, wR2¼ 13.18 %), and a second crystal with composition ZrAs 1.420(3) Se 0.560(1) was investigated at 25 K (R1 ¼2.70%, wR2 ¼5.70 %) and 2.3 K (R1 ¼ 2.30 %, wR2¼ 4.70 %), respectively. The chemical compositions of the crystals under investigation were determined by wavelength dispersive X-ray spectroscopy. The quantification of trace elements was carried out by Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry. According to the crystal structure refinements the crystallographic 2a site is occupied by As, together with a significant amount of vacancies. One of the 2c sites is fully occupied by As and Se (random distribution). With respect to the fractional coordinates of the atoms, the crystal structure determinations based on the data obtained at 25.0 K and 2.3 K did not show significant deviations from ambient temperature results. The temperature dependence of the displacement parameters indicates a static displacement of As on the 2a sites (located on the (0 0 1) planes) for all temperatures. No indications for any occupation of interstitial sites or the presence of vacancies on the Zr (2a) site were found.