Forest fires have been an important source of economic losses in Portuguese municipalities. This work is one of the first studies to test a large range of socioeconomic determinants to explain the differences in the burnt forest areas observed in 278 Portuguese municipalities between 2000 and 2011. Using Classification and Regression Trees, the amount of municipal burnt area per forest fire was observed to depend on the economic dynamism of each locality, the population density of a municipality, the availability of trained teams of forest firefighters, and the presence of relatively high municipal expenditures on environment outlays. We also studied the number of forest fires, concluding that the frequency of forest fires depends on the aging index, the economic dynamism and the average altitude of a municipality.