The roughness of the sea surface has been investigated and described by many researchers. Up to now nearly all researchers considered the roughness to be a function of at least the wind speed and possibly of parameters which are related to the wave field such as significant wave height or wave age. This study is an investigation of how far the equilibrium range of the wave spectrum described by a characteristic parameter such as the Phillips' constant alpha(p) can define the roughness of wind waves. Comparing the Donelan and Toba expressions for the description of a wave spectrum, one could conclude that the Donelan parameter alpha(D), a parameter similar to the Phillips' constant alpha(p), is proportional to the nondimensional roughness of the waves. The KNMI-HEXOS data set is used to verify the hypothesis. It is concluded that the measurements do not contradict the suggested relationship between the nondimensional roughness and the Donelan parameter alpha(D). Linear regression however suggests a stronger than linear dependence of the nondimensional roughness on the Donelan parameter.