The effect of poly(ethylene glycol) (PEG) on the nucleation phase of copper electrodeposition is investigated on three substrates: glassy carbon, as-received ruthenium, and ruthenium that has been pretreated by evolving hydrogen on its surface in diluted sulfuric acid. Microscopy is used to characterize the effect of PEG concentration on nucleus density and the distribution of particle sizes. Results (comparisons are made at the same current density) show that 300 ppm PEG increases the nucleus density by approximately a factor of 6-8 on all three substrates over that obtained without PEG. Images suggest that progressive nucleation is observed on all surfaces. When the PEG concentration is high, the fraction of smaller-sized nuclei also appears to increase. Simulation results are also shown, and it is concluded that PEG impacts nucleation rate, in addition to its well-known effect on growth rate.