We discovered that the parameter defined as the ratio of the fraction of zinc atoms at lattice positions in ZnO powders from the total zinc content (calculated using XPS) to the square of the nanocrystallite size (calculated using XRD) presents a very strong positive correlation (rxy = 0.99) with the rate constant of photocatalytic decomposition of paracetamol in UV radiation. ZnO samples were derived by various chemical methods under various process conditions.