The use of historical information is acknowledged to improve frequency analysis. However, it is not well known if the degree of this improvement depends on the characteristics of both the catch-ment and the data series. In order to address this issue, frequency analyses with historical and systematic data were performed in three very different catchments: the Ondara River in Tàrrega, the Ebro River in Zaragoza and the Ebro River in Tortosa. Besides differences in their physical characteristics, these catch-ments differed also in their data series: their overall lengths and those of the historical and systematic periods.The results show that frequency analysis with historical information is extremely useful in ungauged catchments such as the Ondara River in Tàrrega, because the alternative (rainfall-runoff modelling) gives less accurate estimation of expected peak f lows. However, in gauged catchments (Ebro River in Zaragoza and Ebro River in Tortosa), the degree of modification of peak f low estimates due to the use of historical data depends on the length of the historical period relative to that of the systematic record.