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One of the important subjects in the material science of semiconductor compounds is a problem of stoichiometry. The methods based on the measurement of kinematical integral reflectivity (IR) , R. , for the quasi-forbidden reflection (QFR) of X-rays were formerly used for investigation of the GaAs composition. To determine the value of deviation from stoichiomerry, A =C A-C B where C t is a concentration of the component A or B, one should make some corrections for extinction phenomena which is difficult to take into account for a real (with defects) crystal. The reflectivity of a real crystal for a QFR, R t , may be also described within the dynamical theory taking into account the Debye-Waller static factor , L H , the extinction coefficient (ii ds , and parameter A. To determine these characteristics the experimental thickness pendulum oscillation of Rf (t) (200 reflection, A,=0.1198nm) was measured and analysed for the first time. By this the Honl corrections of atomic formfactors for anomalous dispersion were made. Another independent approach consisted in analysis of the experimental energetic dependence of the R.(A) for the wavelengths situated between the two absorption K-edges was used too. Relatively close values of the L H and (ii ds for GaAs crystals with dislocations as well as parameter A were obtained by fitting of the R. (/) and R t (A) nonlinear functions, calculated by the theory , to the experimental data.

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