This paper is devoted to test the validity of the Eliashberg – McMillan (EM) model and the recent Gor’kov – Kresin (GK) model developed to solve the paradox in the original BCS theory of superconductivity. These models were designed for predicting the isotope effects of some superconducting materials that have a strong coupling coefficient λ ˃ 1.5. The study is focused on new superconducting materials whose critical temperature is close to room temperature, specifically YH6 – YD6 and Th-H – Th-D systems. Although the isotopic coefficient α- values of the two models are close to each other, they deviate from the experimental value with an error percent of 20%. These models, despite their importance and simplicity for predicting the isotope effects, can be considered with critical reliability.