We have performed a theoretical study of the cyclotron effective mass and electron effective Landé g‖ factor in semiconductor GaAs–Ga1−xAlxAs quantum wells under an applied magnetic field parallel to the growth direction of the quantum well. The theoretical approach is within the nonparabolic and effective-mass approximation and via an Ogg-McCombe effective Hamiltonian [Proc. Phys. Soc. London 89, 431 (1969); Phys. Rev. 181, 1206 (1969)] for the electron in the conduction band of the GaAs–Ga1−xAlxAs heterostructure, which allows a unified treatment of both the cyclotron mass and g‖ factor. Calculations are performed for different widths of the GaAs–Ga1−xAlxAs quantum wells and as functions of the applied magnetic field, with results in very good agreement with reported experimental measurements of the electron cyclotron effective mass and g‖ factor.