The composition of MBE-grown GaAs1−xSbx (x<0.15) is investigated as a function of growth temperature for constant As and Sb flux and as a function of Sb effusion cell temperature for a fixed growth temperature. Under conditions of constant As and Sb flux, x remains fairly constant for growth temperatures of 480–540°C, but decreases rapidly with increasing temperature in the range 540–640°C. As a function of Sb effusion cell temperature, the Sb mole fraction is shown to increase slightly slower than the Sb vapour pressure.