[en] The results from regular checks of measuring assembly performance constancy carried out in the period 1981-2000 for five Secondary Standard Chambers are presented. The test procedure consisted of 10 consecutive measurements of collected electrical charge for a time of 200 s each. For all chambers the percentage deviations are into the range of ± 1.5 %. For the whole period the average values of deviations are between 0.15 % and 0.14 %. The results for two chambers (2505/A SN 598 and NPL 2651 SN 096) in chronological succession are graph shown. The histograms of deviations for two other chambers (NE 2571 No. 238 and NE 2575 No. 118 are given. The deviations have a random character and due to dominant influence of some of the affecting quantities on given data. According to the international documents, the performance stability of the Secondary Standard ionization chambers is quite satisfactory