Electron–ion recombination is one of the very important processes in the laser heating of plasmas. With increasing power of the laser fields available, it becomes of interest to evaluate the spectrum of high frequency radiation that can be attained by the recombination process in a powerful laser field. In the present work, we want to reanalyse the efficiency of this process in a radiation field, consisting of up to three laser fields of frequencies rω L, r=1, 2, 3. We choose the intensities and relative phases of these harmonics such that the power spectrum of X-rays is optimized for the highest frequency. The results obtained agree with the classical model, developed for this recombination process.