New experimental data are presented on the influence of various non-nuclear factors on the radioactive decay probability λ. Redox processes carried out in ¹³⁹Ce-containing solutions showed that the relative change in λ as measured by the ΔI/I method was [I(CeIV)-I(CeIII)]/Iav = +(1.4±0.6) × 10-4. A modification of this method based on the shift of the "secular" radioactive equilibrium on cooling the MgO(^121mTe) source to 78 K revealed the growth of λ for the nuclear isomer of tellurium by 0.04±0.02%. New experimental data are presented on increase in the γ-radioactivity of Be^123mTe sample, induced by low-temperature collective nuclear superluminescence.