In situ atomic force microscopy (AFM) was used to observe growth morphology during copper electrodeposition on Pt(100) and Pt(111) surfaces from 0.25M CuSO4/0.5M H2SO4 containing 0, 10, and 100 muM benzotriazole (BTA). The Pt(100) crystal was misoriented approximately 2-degrees to give terraces that were approximately 1 mum in length and 25 to 50 angstrom high. Deposit morphology was monitored primarily during growth at current densities between 1 and 15 mA/cm2 under stagnant conditions. In the absence of BTA, copper selectively deposited on the larger step sites rather than on the smaller steps or terrace regions. From solutions containing 10 muM BTA, copper deposited in clumps along the steps. From solutions containing 100 mum BTA, deposition occurred without regard to substrate features. On Pt(111), three-dimensional nucleation of copper was monitored at the nanoscale level during cathodic deposition from 0.2 5M CuSO4/0.5M H2SO4. At overpotentials <70 mV, no nuclei were observed. At 120 and 170 mV overpotential, individual 3-D clusters were observed, and additional clusters nucleated over time. At higher overpotentials (greater-than-or-equal-to 200 mV), uniform nucleation and overlap were observed. The current transients and AFM images recorded during the deposition experiments were compared with theory for 3-D, solution-diffusion-controlled, multiple nucleation with overlap. At high overpotentials, the current transients were consistent with theory, and AFM images indicated that nucleation was three-dimensional with overlap. At low overpotentials, the current transients were not consistent with theory, and AFM images confirmed that nuclei did not overlap.