We have investigated the spin–orbit interaction in strain-free Al 0.1In 0.9Sb/InAs 0.1Sb 0.9 quantum wells with different widths. The weak anti-localization (WAL) effect was clearly observed in the magneto-resistance for all samples. The inelastic scattering time is close to the T −1 law predicted theoretically for electron–electron interaction in the diffusive regime, and the spin–orbit scattering time is temperature independent as expected for the D’yakonov–Perel mechanism. The zero-field spin-splitting energies extracted from these observations were around 1.0 meV. The WAL behavior becomes distinct in samples with wider well width, and the zero-field spin-splitting energy slightly increases with well width. This seems to reflect increase of the carrier concentration.