The spin-exchange optical polarization method was used to remeasure the zero-field hyperfine splittings of atomic hydrogen and atomic tritium. The transmission of circularly polarized rubidium resonance radiation through a flask containing rubidium, atomic hydrogen, or atomic tritium, and a buffer gas was monitored as a function of the frequency of an applied radio-frequency field. Measurements were made for hydrogen in helium, neon, molecular hydrogen, and argon buffer gases and for tritium in neon and argon buffer gases. The values of the hyperflne splittings obtained are DELTA nu (H) = 1,420,405,749.1 plus or minus 6.0 cps; DELTA nu (T) = 1,516,701,476.8 plus or minus 6.0 cps. The assigned limits of error represent the range of disagreement of the zeropressure extrapolations in the different buffer gases. The fractional pressure shifts STApressure shift in cps/mm Hg divided by the hyperfine splitiing in kMc/secl for hydrogen in helium, neon, molecular hydrogen, and argon are 4.80 plus or minus tively. Those for tritium in neon and argon are 3.24 plus or minus 0.03 and - 5.05 plus or minus 0.15, respectively. (auth) Munoz for measuring atomic partial stopping powers ( xi /sub o/) has been extended to the determination of the stopping power xi /submore » 1/ of hydrogen gas for the H/sup +/ component of a hydrogen beam. The protons in a 1-mm dia. beam are held in an orbit which is an arc of a circle of 142-cm radius, and are guided to a 1-mm dia. exit after a path length of 55.8 cm in --50 mu pressure of H/sub 2/ gas. The energy of the beam after leaving the stopping cell is measured by electrostatic deflection. No proton thus measured can have experienced a charge-changing collision in the stopping cell. In preliminary experiments protons were simulated by deuterons of twice the energy and xi /sub 1/ determined for protons of 55, 75, and 93 kev. Finally, an extended series of runs at 42.8 and 54.2 kev were made on xi /sub 1/ , and a similar extended series on xi /sub 0o/, with the apparatus arranged as in the work of Allison et al. These latter measurements for xi /sub 1/, xi , and xi /sub 0/ gave (6.12 plus or minus 0.44; 6.36 plus or minus 0.16; 2.72 plus or minus 0.16) x 10/sup -15/ ev cm/sup 2// atom, and in the same units, for the higher voltage, (6.04 plus or minus tion of the beam is known from other experiments, the charge-changing collisions may be shown to contribute 34% of the total stopping losses. The sum of the losses in kinetic energy in an electron capture and in its subsequent loss is 40 plus or minus 6 ev at42.8 kev and 54 plus or minus 9 ev at 54.2 kev, neglecting charge-changing collisions involving H/sup -/. (auth)« less