[en] Pure shears of CsDSO₄, CsHSO₄ and CsH₂PO₄ belonging to monoclinic crystal system, have been studied for the first time using the methods of diffraction reflection curves and three-crystal X-ray diffractometry. It is shown that the crystals are perfect enough and scattering upon them obeys laws of dynamic theory. Dislocations with the density of approximately 10⁴-10⁵ cm^-2 are the main defects of the structure. For crystals of CsDSO₄ and CsHSO₄ considerable negative deformations δd/d≅-10^-3 in near the surface layers at room temperature were detected. It is shown that the density of dislocations in near the surface layers of CsDSO₄ and CsHSO₄ monocrystals increases in time