Pt films on alumina substrates, both in their as-received and annealed states, have been extensively characterized by slow positron implantation spectroscopy (SPIS). Bulk Pt samples have been investigated by conventional positron annihilation spectroscopy (PAS) as well as by SPIS. A variety of state-of-the-art theoretical calculations have been performed to aid the interpretation of experimental findings. The research shows that a re-interpretation of earlier defect studies of bulk Pt by PAS is required in order to achieve a satisfactory agreement with the present experimental findings and theory.