The envelope-function approach is used to theoretically study the effects of in-plane magnetic fields on the cyclotron effective mass and Lande g(perpendicular to)-factor associated to conduction electrons in single GaAs-(Ga,Al)As quantum wells. Non-parabolic and anisotropy effects are included in the calculations within the Ogg-McCombe effective Hamiltonian to describe the electron states in the semiconductor heterostructure. The electronic structure and both the cyclotron effective mass and Lande g(perpendicular to)-factor were obtained, by expanding the corresponding envelope wave functions in terms of harmonic-oscillator wave functions, as functions of the in-plane magnetic field, cyclotron orbit-center position, and quantum-well widths. This procedure allows us to consider the different terms in the Hamiltonian on equal footing, avoiding therefore the use of approximate methods to obtain the envelope wave functions and the corresponding energy spectrum. Results obtained for the Lande g(perpendicular to)-factor were found in quite good agreement with available experimental measurements.738