Optically active excitons possess spin J = 1 and can precess in the external magnetic field. Usually, carrier spin precession in the external magnetic field is studied by the modulation or four-wave mixing spectroscopies. The high quality of the MBE-grown heterostructure and narrow spectral line of excitons allows us to experimentally study the exciton spin precession by conventional reflectance spectroscopy methods. Detailed analysis of reflectance spectra gives much more quantitative information about the studied physical phenomena.