Abstract 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 X xy occupied state in a 60Åwell to a non-degenerate X z 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 X xy 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.