An important characteristic of a river flow regime type is the time of year when high and low flows are likely to occur. How likely is it, however, to observe an identified seasonal pattern each individual year? Stability is an often neglected property of a flow regime, though shifts in the seasonal behaviour of flows affect both environmental and economic activities. An approach to characterize objectively the stability of a flow regime type, based on the concept of entropy, is presented. The stabilities of river flow maxima and minima are studied separately to investigate their respective contributions to the stability character of a particular regime type. A quantitative “instability index” permits a study of the development of a flow regime's stability in time, especially important in the context of a possible climate change. The method is presented using the example of a quantitative flow regime classification developed for Scandinavia and western Europe.