Ni superconformal filling of sub-micrometer trenches is demonstrated using a Watts bath containing an inhibitor 2-mercaptobenzimidazole (MBI). Hysteretic voltammetry marks the breakdown of the MBI-induced passive-state coincident with the onset of Ni deposition. Chronoamperometry reveals that disruption of MBI inhibition is a strong function of the immersion conditions and potential conditioning. The passive to active transition involves a competition between potential-dependent MBI adsorption and its subsequent deactivation and/or consumption during Ni deposition. Breakdown of the MBI-passivating layer results in sulfide formation and its incorporation into the Ni film. The current efficiency of Ni deposition on the activated surface is close to that for the additive-free electrolyte. For recessed surface features, such as trenches, inhibition-breakdown initiates preferentially at the bottom corners. Scanning and transmission electron microscopy (SEM/TEM) studies show the growth front within the trenches initially develops as a v-notch shape while negligible deposition occurs on the free surface. This is followed by the onset of Ni deposition over the entire surface profile that results in trench filling by geometrical leveling. Depending on the specimen immersion process a distinct microstructural transition marks the change in growth mode from a superconformal deposition mode to geometrical leveling during trench filling. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3572055] All rights reserved.