Abstract: A method is provided for determining quality of semen of a vertebrate animal. The method includes the following steps: measuring at least one absorption spectrum of a sample of the semen; selecting a number n of wave numbers ?j (j?[1;n]) which are characteristic of the semen of the breed or of the species of the animal; determining from the absorption spectrum or spectra of a value of the absorption Xj and/or a value of the second derivative of the absorption Xj?(j?[1;n]) for each of the n wave numbers ?j (j?[1;n]); and calculating a non-return rate Y at a predefined number of days from the absorption values Xj and/or from the second derivative of the absorption Xj? previously determined.