In this paper, we show that Au nanoparticles (AuNPs) stabilized with either citrate or by low-generation dendrimers rapidly grow during electrocatalytic reduction of CO2. For example, citrate-stabilized AuNPs and AuNPs encapsulated within sixth-generation, hydroxyl-terminated, poly­(amidoamine) dendrimers (G6-OH DENs) having diameters of ∼2 nm grow substantially in size (to 6–7 nm) and polydispersity during just 15 min of electrolysis at −0.80 V (vs RHE). This degree of instability makes it impossible to correlate the structure of AuNPs determined prior to electrocatalysis to their catalytic function. In contrast to the G6-OH dendrimer, the higher generation G8-OH analogue stabilizes AuNPs under the same conditions that lead to instability of the other two materials. More specifically, G8-OH DENs having an initial size of 1.7 ± 0.3 nm increase to only 2.2 ± 0.5 nm during electrolysis in 0.10 M NaHCO3 at −0.80 V (vs RHE). Even when the electrolysis is carried out at −1.20 V, the higher-generation dendrimer stabilizes encapsulated AuNPs. This is presumably due to the compactness of the periphery of the G8-OH dendrimer. Although the G8-OH dendrimer nearly eliminates AuNP growth, the surface of the AuNP is still accessible for electrocatalytic reactions. The smaller, more stable G8-OH DENs strongly favor formation of H2 over CO. Some previous reports have suggested that AuNPs in the ∼2 nm size range yield primarily CO, but we believe these findings are a consequence of the growth of the AuNPs during catalysis and do not reflect the true function of ∼2 nm AuNPs.