Despite the potential of chlorine isotopes to trace inputs of fluids from the subducting slab, few studies have used this tool so far to investigate the petrogenesis of arc magmas. Here we report stable chlorine isotope data (δ37Cl values) and Cl concentrations of volcanic rocks from five Quaternary volcanoes of Ecuador situated on an across-arc transect encompassing the frontal arc (Pilavo, Pululahua, Pichincha volcanoes), main arc (Chacana caldera complex) and back-arc (Sumaco volcano). We find that changes in δ37Cl values across the arc correlate with slab fluid indices (Ba/La, Pb/Ce). The overall decrease in δ37Cl values away from the trench can be interpreted in the frame of previous petrogenetic models of Ecuadorian volcanoes, according to which magmas are formed by a steadily decreasing melt fraction of the mantle induced by a steadily decreasing amount of fluids released by the subducted slab away from the trench. The high δ37Cl values of the frontal arc volcanoes (up to +3‰) imply that the Cl carried by slab fluids derives, at least partly, from 37Cl-rich subducted terrigenous sediments plus subordinate amounts of altered oceanic crust and serpentinite. The anomalously high Cl contents (up to 0.2 wt.% Cl) and high δ37Cl values (up to +1.5‰) of the back-arc Sumaco volcano can be explained by preferential partial melting of mantle portions metasomatized by slab fluids during a Jurassic subduction event. Superimposed on the first order changes of δ37Cl values across the arc, we observe that δ37Cl values at each volcanic center are systematically lowered due to intracrustal evolution processes occurring in magmatic reservoirs at mid-crustal levels.