Materials of nanoscale thickness are widely being explored as strategies for enhancing the performance and stability of photoelectrochemical cells for solar energy conversion. 2D materials such as graphene and transition metal dichalcogenides have shown promise as protecting layers and catalysts for photoelectrochemical cells. Atomic layer deposition techniques can be used to deposit metal oxides that function as ultrathin catalysts or protecting layers. While approaches using 2D materials or ALD often show improved performance, the challenge of analyzing nanoscale materials in operando often means that there is little direct evidence of why ultrathin layers can profoundly differ in catalytic performance or why protecting layers suddenly and catastrophically fail. Here, we present ongoing examples from our lab in which we are using recently developed techniques based on scanning electrochemical microscopy to mechanistically explore mechanisms of catalysis and corrosion on operating photocatalysts. Using microscale anodic stripping voltammetry, scanning electrochemical cell microscopy, and surface interrogations alongside traditional scanning electrochemical microscopy, we are elucidating the structure̶ function relationships governing the performance of both 2D materials and ALD layers at the semiconductor/electrolyte interface. By understanding how ultrathin materials behave differently from bulk materials, our conclusions can facilitate their integration into photoelectrochemical cells.