Analysis of time stability of the profile of(1 1 1)Cu diffraction line in an activated carrier catalyst CHEROX 46-00 has been used in testing the quality of a cuvette for XRD analysis of samples containing a component unstable in air. It was found that the described type of cuvette can provide a distorted information about the size of primary crystallites of analyzed phase if the time stability of sample is not known. New method for protecting the active component of sample in the cuvette against air oxygen was worked out and experimentally tested. The method provides a high time stability of this type of samples for XRD analysis. It consists in the surface protection of an unstable solid phase with a laminar liquid film of a suitable non-reactive sorbate (e.g., 2-ethylhexanol) and in the analysis of sample in a pasty form. The efficiency of sample stabilization is a function of the content of dispersing liquid in the analyzed paste.