Numerical results are presented for the diurnal variations of stratospheric species, including the chlorine species. The calculated diurnal variation of the column density of stratospheric chlorine monoxide (ClO) appears to be in good agreement with the recent measurement obtained from ground-based millimeter-wave spectroscopy. The observed large diurnal variation of the column density of ClO above 30 km can be understood in terms of various exchange processes ClOx (Cl+ClO) and the temporary chlorine reservoirs (CiONO2 and HOCl). The rapid early morning rise in the column density of ClO above 30 km is mainly attributed to photolysis of ClONO2, whereas its continual decrease during the nightime may be attributed to the recombination of ClO with NO2, forming ClONO2 as the major product. The predicted diurnal behavior of the upper stratospheric ClO, however, is mainly due to the exchange between ClOx and HOCl. Observation of the diurnal behavior of ClO in the upper stratospheric can place important constraints on the chemistry of HOCl.