Photoabsorption cross sections of HCl and DCl in the spectral region 120-220 nm were measured using synchrotron radiation as a light source. The absorption contour of the A (1)Pi<--X (1)Sigma(+) band of HCl in the 134-195 nm region is slightly asymmetric, whereas that of DCl is symmetric, with wavelength of absorption maximum shifted from similar to153.9 nm for HCl to similar to155.8 nm. The asymmetry in HCl and the atypical direction of shift in peak wavelengths upon deuteration are attributed to singlet-triplet mixing in its ground X (1)Sigma(+) state of HCl; this triplet component contributes to the oscillator strengths of the A-X band of HCl via transition to the repulsive triplet state 1 (3)Sigma(+). The small triplet character in HCl is further supported by an observation that the oscillator strength of the (0,0) band for the b (3)Pi-X (1)Sigma(+) transition of DCl is similar to17% smaller than that of HCl. This triplet component may account for greater values of spin-orbit branching ratios for products, Cl-*(P-2(1/2))/Cl(P-2(3/2)), observed upon photolysis of HCl in the 143-167 nm region. For both HCl and DCl, ratios of observed oscillator strengths to calculated Franck-Condon factors of the C (1)Pi state decrease with vibrational levels, whereas those of the b (3)Pi state increases with increasing vibrational levels. Such a variation was attributed to perturbation between the b and C states, for which quantitative data are established in this study.