Neutral beams are used to fuel and heat the Tandem Mirror Experiment Upgrade (TMX–U) at LLNL. We have studied these beams spectroscopically to determine their species mix (H, H2, H+3) and impurity content (H2O). From the shapes of the spectral lines, we measured the divergences of the various components and compared them with the numerically determined divergence. By measuring the area under the various Doppler‐shifted lines, we determined the species mix and found that the atomic species fraction is lower than expected (30% to 45% rather than 60%). Although there is good agreement between the measured and calculated divergence for the atomic component of the beam, the molecular components have larger divergences, part of which can be accounted for by the dissociation of the molecules.