We analyze, via a perturbative calculation, the two-photon ionization of hy
drogen atom by a coherent superposition of a laser field and three of its o
dd harmonics of orders 2p + 1, 4p + 1, and 4p + 3 with p a positive integer
. Numerical results reveal a strong dependence of the total ionization rate
on the relative phases of the harmonics with respect to the fundamental. E
ffects connected with the photon polarizations are also investigated.