The temperature dependence of the plateau width ∆ Bi in heterostructures GaAs-Ga 1- xAl xAs has been investigated in the integral quantum Hall effect (IQHE) regime. The measurements have been made on samples with electron density N=(2.5-5)×10 15m-2 and mobility μ=(30-80) m2Vs-1 (measured at T=4.2 K) in the temperature range (0.08-4.2) K in magnetic fields B up to 8.5 T. Analytical expressions for the dependence ∆ Bi( T) have been obtained. The average values ( gi) of the g-factor for 2D-electrons which correspond to quantum plateaux with various numbers i have been calculated. It has been found that gi strongly depends on the magnetic field: it rapidly increases in weak fields, and tends to be saturated in high ones. In the field range (2-5) T, the g-factor increases by more than a factor of 1.6. The dependence g( B) found is consistent with the idea of the exchange interaction between the electrons occupying the neighbouring Landau subbands.