Alpha-particle emission probabilities in the decay of 238U

238U decays by alpha-particle emission to 234Th. No direct measurements of alpha-particle emission probabilities (Palpha) of this nuclide have been reported since 1961, and recommended values for Palpha have remained unchanged for years, until recent evaluations suggested new values. This work presents the results of new measurements made with Si detectors and sources of natural uranium. The results obtained for the analysis of twenty spectra are: Palpha0 = 0.7754+/-0.0050, Palpha50 = 0.2233+/-0.0050 and Palpha163 = 0.0013+/-0.0003.