Abstract. Double transport barrier plasmas comprised of an edge EDA H-mode pedestal and an ITB have been observed in Alcator C-Mod. The ITB can be routinely produced in ICRF heated plasmas by locating the wave resonance off-axis near r/a 0.5, provided the target plasma average density is above 1.4×1020m3, and can develop spontaneously in some Ohmic H-mode discharges. The formation of the barrier appears in conjunction with a decrease or reversal in the central (impurity) toroidal rotation velocity. The ITBs can persist for 15 energy confinement times, but exhibit a continuous increase of the central electron density, (in the absence of an internal particle source), followed by collapse of the barrier. A significant drop of the core thermal conductivity when the barrier forms is confirmed by modeling. Application of additional on-axis ICRF heating arrests the density and impurity peaking, which occurs along with an increase (co-current) in the core rotation velocity. The density peaking is found to be consistent with an inward neoclassical pinch velocity and a reduced particle diffusivity. Linear growth rate calculations indicate the ITG mode is stabilized in the barrier region.