Photoluminescence (PL) spectra and intensities of thin NN'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (alpha-NPD) films have been measured at room temperature, during sample heating with various rates and annealing times at constant temperatures, and after annealing. It was found that the temperature of T = 80 degrees C being considerably lower than the glass transition temperature of alpha-NPD is sufficient to cause substantial irreversible changes in PL and PL excitation characteristics. A PL efficiency increase up to 10 times, an emission spectrum short-wavelength shift up to 130 meV and a spectral narrowing from 69 to 39 nm are reached using annealing. The surface roughness of the films annealed at the moderate temperature of 80 degrees C does not undergo observable changes contrary to films annealed at higher temperatures. (c) 2006 Elsevier B.V. All rights reserved.