Given its concentration in a few days and its modest total volume, knowledge about precipitation with daily resolution in Spain is highly important. In this article, a concentration index (CI) that evaluates the varying weight of daily precipitation, ie the contribution of the days of greatest rainfall to the total amount, is presented. The index is applied to exponential curves such as Y= aX exp (bX), which adjust the accumulated percentages of precipitation Y contributed by the accumulated percentage of days X on which it took place. The index was applied to 32 meteorological stations across peninsular Spain with quality data for the period 1951 to 1990, which enables the spatial patterns of daily pluviometric concentration in the territory to be determined. Copyright 2004 Royal Meteorological Society.