A zonally averaged photochemical--dynamical model of the middle atmosphere is used to simulate the quasi-biennial oscillation (QBO) and its effect on the distributions of stratospheric H2O, CH4, and age of air. Changes in planetary wave amplitudes of ¿25% cause 2--3 month changes in QBO period. Comparable changes in prescribed tropical heating have a smaller effect on the QBO period. The response of tropical upwelling, and QBO period, to changes in extratropical forcing depends on the magnitude and location of the imposed changes. In the Southern Hemisphere, where the planetary wave forcing is smaller than in the Northern Hemisphere, increased forcing produces stronger equatorial upwelling and a longer QBO period. In the Northern Hemisphere, increased forcing produces weaker upwelling and a shorter QBO period due to the larger amplitude waves becoming saturated. Overall, the effect of the QBO is to produce a slightly younger mean age of air near the tropical stratopause due to the model not exactly reproducing the strength and duration of the observed westerly wind shear. The QBO in lower stratospheric H2O, but not CH4, results primarily from meridional circulation anomalies superimposed upon background gradients in H2O mixing ratio that are maintained by the annual cycle in lower stratospheric H2O. QBO modulation of horizontal eddy transport plays a much smaller role. A realistic annual cycle in equatorial H2O mixing ratio at the model tropopause produces a QBO variation in lower stratospheric H2O of 0.1--0.2 ppmv. The effect of the QBO on model tropical tropopause temperatures doubles the amplitude of the water vapor QBO at 20 km.