We have studied the spin splitting of Landau levels for a two-dimensional electron gas in an InAs/GaSb single quantum well. The effective g factor, g*, is between 7.8 and 8.7 which is below the bulk value. The reduction in g* is due to nonparabolicity effects resulting from confinement of free electrons to a narrow quantum well. Under these circumstances g* enhancement due to many-body effects is smaller than the reduction due to nonparabolicity. © 1987 The American Physical Society.