We have performed a theoretical study of the cyclotron effective mass and electron effective Lande g(parallel to) 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(parallel to) 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(parallel to) factor. (c) 2006 American Institute of Physics.9910