Volatile emissions from arc-related volcanic and hydrothermal systems are characterized by elevated CO 2 /He, N 2 /He and N 2 /Ar ratios compared to MORB. The source of CO 2 and N 2 is thought to be subducted sediments and oceanic crust, whereas helium is mainly derived from the mantle wedge and Ar is contributed from the atmosphere. The CO 2 and H 2 O concentrations of arc-related magmas have been measured in melt inclusions from explosively erupted tephra and suggest that arc-magmas have CO 2 contents of up to ∼2000 ppm and H 2 O contents up to ∼6 wt.%, supporting the idea of elevated volatile contents in the sub-arc mantle compared to MORB. We present here a new approach to estimating the volatile content of primary undegassed sub-arc magmas taking advantage of the fact that 3 He originates solely from the asthenospheric mantle wedge. Recent estimates of 3 He flux from arcs, 3 He concentration in the crystalline mantle, and arc S, CO 2 , N 2 , and Ar fluxes allow to estimate concentrations of these volatiles in primitive undegassed sub-arc melts. Our results suggest ∼8 to 16 wt.% H 2 O, ∼500 to 2200 ppm S, ∼3500 to ∼7600 ppm CO 2 , ∼11 to 107 ppm N 2 , 0.1 to 0.3 ppm Ar for undifferentiated melts produced by 20% partial melting of the crystalline mantle wedge. Although the uncertainties are large, the results for H 2 O and CO 2 are generally higher than what has previously been measured in olivine-hosted melt inclusions from arcs. We reconcile this difference being due to deep volatile degassing of sub-arc magmas prior to melt inclusion entrapment at shallower levels in the crust.