We present time-resolved differential-absorption data for GaAs quantum wells, in which temporal oscillations originating in a biexcitonic pairing appear. We interpret the oscillations within a four-level model of the exciton-biexciton system, and discuss the cases of homogeneously and inhomogeneously broadened exciton absorption lines. Our differential absorption measurements in high magnetic fields normal to the quantum wells layers show that both the oscillations frequency and phase depend on the strength and direction of the magnetic field. The frequency change corresponds to the exciton Zeeman splitting in our samples. These findings are in excellent agreement with the predictions of the model, and thus verify its validity. © 1994 The American Physical Society.