Abstract <p>Temporal variations of nuclear decay parameters actively studied now; their detection may be a signal of new physical effects of cosmic origin, in particular, due to influence of solar processes. In some recent experiments annual and daily oscillations of unstable nuclei α- and β-decay parameters were observed at a level about 0.05%. Correlations of intensity of ⁶⁰Co, ⁵⁴Mn and ⁵⁵Fe weak decays with solar activity, in particular with solar flares, were observed. In our work, correlations of X-class solar flares with ⁶⁰Co and ⁵⁵Fe decay rate parameters were studied, their measurement were performed by means of decay γ-ray detection by semiconductor detectors. For ⁵⁵Fe decay eight significant deviations of decay counting rate in form of reduction from the expected rate were observed at the level ~0.2% correlated with solar flares. For ⁶⁰Co decay four similar events were detected with deviations at a level of ~0.4%. All decay rate deviations have advanced character and begin from 30 to 155 h before solar flare moments with a confidence level of 90%.</p>