The spectrum of the Doppler shifted hydrogen Balmer-a emission of the JET neutral heating beams is used to determine their divergence in situ during plasma operation. The line widths have been measured in the presence of the motional Stark effect, employing a numerical fit procedure which makes use of the detailed knowledge of the atomic physics involved. The dependence of the line width on the beam divergence is discussed. The beam divergence is inferred for the various beam operating conditions covering an energy range from 25 to 70keV/amu and all three hydrogen isotopes. The in situ results are compared to testbed measurements.