A technique for controlling the magnitude and phase of the co-to-cross reflection coefficients in microstrip reflectarrays cells is presented. It is based on truncating different patch corners, and thus can be efficiently applied to patches with sizes close to the array lattice. The main application of this technique is a novel synthesis method for reflectarrays with quasi-null cross-polarization, which requires the control of the four terms of the cells scattering matrix. However similar strategies could also be of use in other surface-based antennas and optical devices.