|Single crystal X-ray diffraction studies of two inorganic N-oxide complexes with copper(II) salts and two biologically active compounds have been undertaken. All three-dimensional intensity data were collected using a Philips PAILRED diffractometer. The copper complexes were solved using the Patterson heavy atom method and the biologically active compounds were solved using direct crystallographic methods. The 1:1 complex of pyridine 1-oxide and copper(II) bromide, [(C5H5NO)CuBr2]2, belongs to space group P2_1/c. The structure was refined by least-squares methods to a conventional R-factor of 0.072 for 759 observed intensities. The complex is an oxygen-bridged dimer and has a bromide ion from an adjacent molecule in an apical site above the copper atom, Cu-Br" = 3.130Å. The geometry about the copper atom is best described as highly distorted square-based pyramid. The 4:1 complex of 2,6-Lutidine 1-oxide and copper(II) perchlorate, [(C7H9NO)4Cu(ClO4)2], belongs to space group I4/m. The organic ligands lie in four-fold "swastika" like coordination about the copper atom. The perchlorate ions are in four-fold disorder above and below the copper atom in the axial sites. The structure was refined by least-squares techniques to a conventional R-factor of 0.125 for 569 observed intensities. The nitrogen mustard, 1-(2-Chloroethyl)-3-(4-carbamoylpyrazol-3-yl)-Delta2-1,2,3-triazolinium chloride, [C8H12N6OCl]+Cl-, belongs to the space group Pbca. The structure was refined by least-squares techniques to a final R-factor of 0.0497 for 1043 observed reflections. The compound is of interest in anti-tumor research. It is inactive in the solid state after undergoing ring closure to form the triazolinium salt. One of the isolated photoproducts resulting from the anerobic ultraviolet irradiation of plastoquinone-1 determined was to be the dimer 5,6,6a,7,7a,11b-hexahydro-2,3,5,5,7,7,9,10-octamethyl-6,11a,11c-metheno- 1H-benzo[c]fluorene-1,4,8,11(4aH)-tetrone, C26H30O4. The compound belongs to the space group Pccn and was refined by least-squares techniques to a final R-factor of 0.0759 for 2259 observed intensities. Plastoquinone-1 is under study as a model compound in the electron transport processes that occur in photosynthesis in subcellular organelles.