Using small area electrical data logging, high speed photography, sample mass gain monitoring, gas evolution measurement and microstructural examination, a study has been made of the formation and effect of cathodic discharges during PEO of Al substrates. Discharge formation during the cathodic half-cycle is promoted by high frequency, thick coatings and high pH. They form (under constant current conditions) when the voltage during cathodic polarization reaches a value (~250 V in the work described here) sufficient to cause dielectric breakdown across thin residual oxide layers on the substrate. This occurs when the normal cathodic process of proton flow through electrolyte channels in the coating can no longer deliver the required current. Cathodic discharges tend to carry higher currents, and emit more light, than anodic ones. Gas evolution rates during PEO are well above the Faraday yield level. This is due to water entering discharge plasmas, breaking down into ionized species and failing to recombine completely during subsequent collapse and quenching. It is reported here that rates of gas evolution rise as discharges start to take place in the cathodic part of the cycle, as well as in the anodic part. Rates of substrate oxidation (coating growth), however, drop off, rather than rise, when cathodic discharges start. Evidence is presented here suggesting that this is associated with their highly energetic nature, causing substantial amounts of oxide to be expelled into the electrolyte during cathodic discharges. This is also apparent in the coating microstructure, where recent cathodic discharge sites are identifiable as large, highly porous regions.