Accurate measurements of the second-order nonlinearity profile of thermally poled low-loss germanosilicate films grown on fused-silica substrates are reported, of interest as potential electro-optic devices. After optimization, we demonstrate a record high nonlinear coefficient d 33 ≈ 1.6 pm/V, a two-fold improvement over highest reported d 33 value in fused silica that we attribute to the presence of germanium.