Described herein is a method for determining second- order nonlinear optical coefficients of a material. The method envisages the steps of providing a specimen (1;35) made at least in part of the material, causing a first optical signal (Si1) and a second optical signal (Si2) having, respectively, a first pulsation and a second pulsation, and a first polarization state and a second polarization state, to impinge upon the specimen in such a way that the specimen generates a second-harmonic optical signal (su3) having a third pulsation equal to the sum of the first and second pulsations, and a third polarization state that is a function of the first and second polarization states. The method further envisages the step of determining a plurality of measurements of power corresponding to the second-harmonic optical signal, and the step of determining the second-order nonlinear optical coefficients on the basis of the plurality of measurements of power. In addition, the method envisages performing a plurality of measurements of power of the second-harmonic optical signal as the first polarization state and the second polarization state vary.