To develop a new method to determine the work function (phi) of powdery samples, saline hydride deposited on a molybdenum ribbon was heated up to similar to 1000 K at a constant rate in vacuum. The thermal electron current directly emitted from the hydride was measured simultaneously with the ion current of H-2(+) detected by mass spectrometry of H-2 liberated from the hydride. Theoretical analysis of temperature-programmed desorption spectra thus obtained yields that (1) phi of NaH at similar to 740-800 K is 2.3 +/- 0.1 eV, where the percentage of thermal decomposition (NaH --> Na+1/2H(2)) creating active spot (Na) is similar to5-20%, (2) phi of NaH increases by up to similar to0.1 eV with a temperature increase up to similar to 870 K (similar to 80%), and (3) phi of LiH at similar to 860-900 K (similar to 60-65%) is 5.5 +/- 0.5 eV, but this value is increased by up to similar to0.2 eV by a temperature increase up to similar to 940 K (similar to 80%). In conclusion, our method makes it possible to determine phi as a function of the decomposition percentage.