We have performed magnetoresistance measurements up to 20 T on high-mobility two-dimensional electron gases in single AlAs quantum wells with well widths ranging from 40 to 80Å. We observe a sharp transition from a two-fold valley degenerate Xxy occupied state in a 60Åwell to a non-degenerate Xz occupied state in a 40Åwell. These two states are distinguished by their different effective masses, which have been determined using the temperature dependence of the Shubnikov-de Haas effect. Using self-consistent calculations, we can explain this transition in relative occupation assuming an X-splitting energy of 24 meV. Furthermore, for the Xxy electrons, the effect of the large ratio of spin to Landau splitting is shown by the occurrence of strong magnetoresistance minima at filling factors 2, 6 and 10 in the Hall effect.