Nowadays, separate collection plays an irreplaceable role in municipal solid waste management and incineration in a waste-to-energy plant is one of the main disposal options. The objective of this study is to evaluate the effect of separate collection on the characteristics of residual municipal solid waste in terms of lower calorific value and ash production. Starting from the average Italian municipal solid waste composition, several separate collection scenarios have been considered evaluating the main characteristics of the residual waste (e.g. composition, water content, lower calorific value). Moreover, in order to have the opportunity of estimating the lower calorific value of residual waste for waste composition and separate collection scenarios different from those tested, a regression model is proposed, calibrated and partially validated.