The quality of lattice-matched ZnxCdyMg1-x-ySe epitaxial layers and related quantum well laser structures grown on (001) InP substrates has been improved by using Zn irradiation of the III-V surface before the growth of II-VI materials. The full widths at half maximum of the double crystal x-ray rocking curves for ZnxCdyMg1-x-ySe epilayers with band gaps as high as 3.0 eV were reduced to about 50 arcsec, The defect density evaluated from the etch pit density and cathodoluminescence measurements was reduced from 10(6) to mid-10(4) cm(-2). This result suggests that an initial reaction between Se and group-III (In and Ga) atoms is a main limit to the quality of ZnxCdyMg1-x-ySe grown on InP and the problem can be suppressed by using initial Zn irradiation of the III-V surface.