Abstract
New results of ab initio Floquet calculations of rates for multiphoton ionization of atomic hydrogen by a coherent superposition of an intense laser field and one of its harmonics (second, third, fifth or seventh) are presented. The authors discuss further the relationship of these processes with two-colour ionization by fields with incommensurable frequencies, as well as the range of validity of Perelomov and Popov's formula (1967).