Basin-scale transport of reactive solute species is studied through a class of stochastic models, termed mass response functions, which incorporate simplified concepts of chemical, physical or biological nonequilibrium kinetics into the theory of the hydrologic response. Here we examine the development of the field since its inception dealing with empirical approaches, a subject to which Giuseppe Bendoricchio actively contributed, and conclude that a coherent theoretical framework now exists that allows to address large-scale transport problems for catchment studies where geomorphological and hydrological complexity is not simply ignored. © 2004 Elsevier B.V. All rights reserved.