Thermal helium desorption spectrometry has been used to study the interaction of helium with defects in Cu films (5-300 angstrom) deposited on Ta(110) and Ta(100) single crystals by ultrahigh vacuum electron beam evaporation. The thermal stability of the Cu films was also investigated. Cu films on Ta(110) and Ta(100) at room temperature are metastable and on heating, the films transform into islands. The temperature at which this takes place is strongly dependent on the Cu film thickness and for a given thickness (>40 angstrom) occurs at a lower temperature on Ta(100) than on Ta(110). The activation energy for island formation is 1.6 +/- 0.4 eV for 50 angstrom Cu/Ta(110) and 0.8 +/- 0.1 eV for 100 angstrom Cu/Ta(100) obtained by Kissinger analysis. The geometry of the Cu islands resulting from annealing 50 angstrom Cu films at 1000 K for 10 s depends strongly on the Ta substrate orientation. There is evidence for the stressed states of both the Cu films and the Ta substrates. Helium release from monovacancies and vacancy clusters in Cu films (>75 angstrom) on Ta(110) and Ta(100) was detected at similar to 750 K and similar to 800-1000 K respectively. The sublimation of the Cu films from the Ta substrates could be observed by the release of retained helium at similar to 1300 K. (C) 2009 Elsevier B.V. All rights reserved.