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We have detected new HD absorption systems at high redshifts, z_abs=2.626 andz_abs=1.777, identified in the spectra of the quasars J0812+3208 and Q1331+170,respectively. Each of these systems consists of two subsystems. The HD columndensities have been determined: log(N(HD),A)=15.70+/-0.07 for z_A=2.626443(2)and log(N(HD),B)=12.98+/-0.22 for z_B=2.626276(2) in the spectrum of J0812+3208and log(N(HD),C)=14.83+/-0.15 for z_C=1.77637(2) and log(N(HD),D)=14.61+/-0.20for z_D=1.77670(3) in the spectrum of Q1331+170. The measured HD/H2 ratio forthree of these subsystems has been found to be considerably higher than itsvalues typical of clouds in our Galaxy. We discuss the problem of determiningthe primordial deuterium abundance, which is most sensitive to the baryondensity of the Universe \Omega_{b}. Using a well-known model for the chemistryof a molecular cloud, we have estimated the isotopic ratioD/H=HD/2H_2=(2.97+/-0.55)x10^{-5} and the corresponding baryon density\Omega_{b}h^2=0.0205^{+0.0025}_{-0.0020}. This value is in good agreement with\Omega_{b}h^2=0.0226^{+0.0006}_{-0.0006} obtained by analyzing the cosmicmicrowave background radiation anisotropy. However, in high-redshift clouds,under conditions of low metallicity and low dust content, hydrogen may beincompletely molecularized even in the case of self-shielding. In thissituation, the HD/2H_2 ratio may not correspond to the actual D/H isotopicratio. We have estimated the cloud molecularization dynamics and the influenceof cosmological evolutionary effects on it.

HD/H2 Molecular Clouds in the Early Universe: The Problem of Primordial Deuterium