To elucidate the thermionic property of polycrystalline surfaces, a further study is made on the mean work functions (phi(-), phi(e) and phi(+)) effective for negative-ionic, electronic and positive-ionic emissions. Comparison between theoretical analyses and experimental data yields the conclusions as follows. (1) The equation of phi(-) = phi(e) holds always with both mono- and polycrystalline surfaces. (2) The relation of phi(-) = phi(e) < phi(+) applies to polycrystalline surfaces because they bear the thermionic contrast (Delta phi* phi(+) - phi(e) > 0). (3) The value of Delta phi* ranges from similar to 0.4 to 0.9 eV depending upon the surface species of polycrystalline metals (e.g., W, Re and Pt), whilst Delta phi* = 0 for monocrystalline surfaces. (4) When the degree of mono-crystallization (delta(m)) is less than similar to 50%, the theoretical value of Delta phi* is virtually independent of 3 and agrees well with experimental data, nearly the same within +/- 0.1 eV among the so-called "polycrystalline" surfaces of W. (5) As delta(m). increases beyond similar to 80 up to 100%, Delta phi* decreases rapidly down to 0 cV, showing again a good agreement between theory and experiment. (6) Our theoretical model is valid in evaluating the effective mean work functions, irrespective of the range of delta(m). (c) 2005 Elsevier B.V. All rights reserved.