The selection of specific elastic response spectra according to soil categories is the easiest way to account for site effects in engineering projects and general-purpose hazard maps. Most of the international seismic codes make use of the average shear wave velocity of the upper 30 m (Vs_,30) to discriminate soil categories, although some doubts arose about the capability of Vs_,30 to predict actual soil amplification. In this work we propose two soil classifications in which the soil fundamental frequency (f₀) becomes either an alternative or a complement to Vs_,30. The performance of the derived categorizations is achieved through the estimation of the standard deviation associated to ground motion prediction equations of acceleration response spectra, considering recordings extracted from the Italian strong motion data base. The results indicate that there is a significant reduction of the standard deviation when the classification is based on the couple of variables Vs_,30–f₀, although a classification based of the single f₀ also leads to satisfactory results, comparable with those obtained assuming a classification scheme based on Vs_,30.