Electrocatalytic and photoelectrocatalytic technologies are key players in the transition to a circular economy. In order to rationally design the new generation of efficient (photo)electrocatalysts it is critical to establish which performance bottlenecks must be overcome during the complex catalytic transformations. Obtaining such detailed mechanistic and kinetic knowledge requires the use of operando spectroscopic techniques that are capable of probing the system under operating conditions. Of all the methods available UV–Vis spectroelectrochemistry stands out for its versatility and experimental simplicity which enables the systematic study of samples under a wide range of reaction conditions. From a mechanistic viewpoint, this technique allows us to quantify the accumulation of reactive species at the catalyst–electrolyte interface and, through a kinetic population model, opens the door to characterising the kinetics of the rate determining step of catalysis. Here, we review the different information that can be extracted from UV–Vis spectroelectrochemistry and how it can be used to understand catalytic mechanisms in solid (photo)electrodes.