On the basis of quantum chemical model IR laser multiphoton absorption probabilities and corresponding isotope effects for collisionless UF6molecules in the electronic ground state are calculated. Taking into account the detailed fine structure of the spectrum of the pumped mode the isotopic selectivity of the multiphoton excitation process of UF6can be proved theoretically. The results are compared with experimental results and suggest a begin of the quasicontinuum states of UF6at a vibrational level n ≧ 6.