Abstract Air filters and fallout samples from the Chernobyl accident have been investigated by β-analysis for the determination of the 137Cs/ 90Sr ratio and the absolute 90Sr activity. Gamma-spectroscopic data from May 1986 have been used for the calculation of the 140Ba/ 137Cs ratio and the absolute 140Ba activity. The observed ratio in the samples is approximately the same as in the core of the damaged reactor. It is concluded that γ-spectroscopic measurements of 140Ba and the reactor core ratio of 140Ba/ 90Sr can be used for the estimation of the strontium activity in the fallout immediately after an accident. The 137Cs/ 90Sr ratio can be used for the estimation of the strontium fallout in the late post-accident period.