Films of aluminium were evaporated on to quartz substrates at pressures of less than 2 × 10 −9 Torr. The photoelectric emission current for vacuum-side illumination by monochromatic light of photon energy in the range 4.3–5.0 eV was determined as a function of the thickness of the film. The curve of photocurrent vs film thickness rises to a maximum at a thickness of about 50 Å, and then falls to a constant value of about 0.2 of the maximum. The photoelectric work function increases to a value of 4.27 ± 0.02 eV at a thickness of 50 Å, and remains approximately constant as the thickness of the film is increased. The maximum in the yield curve is due to a maximum in the number of photons absorbed, probably due to scattering of the incident light between the irregular islands making up the film and to the relatively high surface-to-volume ratio of the islands.