Abstract: Cowlesite specimens from nine new occurrences in Northern Ireland and from eight localities previously reported in the literature, were crystal-chemically characterised. The chemical data for a total of 25 samples indicate a composition close to the stoichiometric formula for cowlesite and a very limited compositional range. X-ray powder diffraction spectra were performed on 8 samples. The crystallographic unit cell permit a proper indexing of the X-ray powder diffraction patterns as orthorhombic, unit cell constants close to a 23.3, b 30.6, c 25.0 Å and doubled with respect to those originally reported in literature. The powder pattern and the TG curve are reported for an unidentified mineral species associated with cowlesite in one of the samples from Northern Ireland.