Methods and apparatus for gas delivery are disclosed herein. In some embodiments, a gas delivery system includes an ampoule for storing a precursor in solid or liquid form, a first conduit coupled to the ampoule and having a first end coupled to a first gas source to draw a vapor of the precursor from the ampoule into the first conduit, a second conduit coupled to the first conduit at a first junction located downstream of the ampoule and having a first end coupled to a second gas source and a second end coupled to a process chamber, and a heat source configured to heat the ampoule and at least a first portion of the first conduit from the ampoule to the second conduit and to heat only a second portion of the second conduit, wherein the second portion of the second conduit includes the first junction.