  Observations of light isotopes in cosmic rays provide valuable information on
their origin and propagation in the Galaxy. Using the data collected by the
AMS-01 experiment in the range ~0.2-1.5 GeV/nucleon, we compare the
measurements on 1H, 2H, 3He, and 4He with calculations for interstellar
propagation and solar modulation. These data are described well by a
diffusive-reacceleration model with parameters that match the B/C ratio data,
indicating that He and heavier nuclei such as C-N-O experience similar
propagation histories. Close comparisons are made within the astrophysical
constraints provided by the B/C ratio data and within the nuclear uncertainties
arising from errors in the production cross-section data. The astrophysical
uncertainties are expected to be dramatically reduced by the data upcoming from
AMS-02, so that the nuclear uncertainties will likely represent the most
serious limitation on the reliability of the model predictions. On the other
hand, we find that secondary-to-secondary ratios such as 2H/3He, 6Li/7Li or
10B/11B are barely sensitive to the key propagation parameters and can
represent a useful diagnostic test for the consistency of the calculations.
