Abstract: Antibacterial compositions comprising nanoparticles formed from copper oxo-hydroxide are described that are capable of delivering biocidal concentrations of copper, typically in the form of free copper ions (Cu2+). The nanoparticle compositions generally comprise small particles, typically having mean diameters in the range of 1-100 nm, having comparatively high surface area-to-volume ratio and enhanced reactivity compared to the corresponding bulk counterpart materials and which are sufficiently labile to release the free copper efficiently.