Calcium isotopic compositions (δ44 / 42Ca) were measured in Icelandic rivers draining a range of catchment types. The δ44 / 42Cavalues of the rivers ranged from 0.45‰ to 0.67‰, which in all cases was higher than the δ44 / 42Ca value of basaltic rock standards (0.42±0.03‰). A single explanation was unable to satisfactorily explain the δ44 / 42Ca values of all rivers, rather it was found that the rivers formed three distinct groups based on the extent of glacial coverage in each catchment. The Ca isotopic composition of rivers draining catchments with less than 10% glacial cover could be explained by the mixing of water sources: basalt-derived solutes, meltwater (taken to represent meteorological precipitation inputs) and hydrothermal water. However, fractionation of δ44 / 42Ca in these catchments cannot unequivocally be ruled out. In catchments with greater than 22% glacial cover, Ca isotopic compositions could not be explained by a mixture of water sources and instead reflected a fractionation process, most likely the precipitation of Ca-bearing secondary minerals or the adsorption/ion-exchange of Ca onto mineral surfaces. The fractionation factor (α) for this process was calculated to be 0.9999. The third group of rivers, with partially glaciated (10-21%) catchments, grouped with glaciated catchments with respect to their Sr geochemistry and with non-glaciated catchments with respect to their Ca geochemistry. The difference in the controls of Ca isotope fractionation between glaciated and unglaciated catchments was attributed to different weathering regimes.© 2013 Elsevier B.V.