Mixed anion systems are of current interest due to their unusual properties. This has been highlighted recently by reports of p-type semiconducting behaviour in transparent LaCuOS1, and of iron-based superconductivity in LnFeAs(O,F) materials2. Both these classes of materials adopt the ZrCuSiAs structure and I’d like to present recent work that extends this family to include a number of transition metal oxyselenide phases with ZrCuSiAs-related structures. I’ll discuss how we can get the most from our powder diffraction data to understand these complex crystal structures, and how the structure influences their physical (e.g. optical, electronic and magnetic) properties. 1. Ueda, K.; Inoue, S.; Hirose, S.; Kawazoe, H.; Hosono, H., Appl. Phys. Lett. 2000, 77 (17), 2701-2703. 2. Kamihara, Y.; Watanabe, T.; Hirano, M.; Hosono, H., J. Am. Chem. Soc. 2008, 130, 3296-3297.