Seasonal concentration in tourist flows is often seen as an undesirable consequence of the tourism industry growth in a tourist destination because of its negative impacts on environment, use of natural resources and infrastructures, quality of life for inhabitants. Local governments can take various initiatives to counter seasonality in tourist flows, such as market diversification or different pricing strategies for distinct tourist segments. To assess the effectiveness of such initiatives, a statistical toolkit for measuring the changes in seasonal concentration is needed. This paper suggests a decomposition that separates the roles played by different segments of tourists in changing the pattern and magnitude of seasonal concentration in tourist flows.