Abstract <p>Temporal variations of nuclei decay parameters are actively studied now; their detection may be a signal of new physical effects of cosmic origin. Correlations of ⁶⁰Co, ⁵⁴Mn, and ⁵⁵Fe weak decay parameters with solar activity, in particular with solar flares, were observed earlier. In our work, correlations of X-class solar flares with ⁶⁰Co and ⁵⁵Fe decay rate variations are measured via decay γ-ray detection by semiconductor detectors. For ⁵⁵Fe decay eight significant deviations of decay counting rate from the expected rate observed at the level ∼0.2% during 48–96 h, correlated with solar flares. For ⁶⁰Co decay four analogous flare events were detected with deviations at the level of ∼0.4%. All decay rate deviations have advanced character and begin from 30 to 155 h before solar flare moments at confidence level of 90%.</p>