Studies of the oxygen ion and cation transport processes during plasma anodization of bare Si and Si through thin ZrO2 films have been carried out using a combination of transport number measurements of cations t+ (ratio of the cationic current to the overall ionic current), isotopic tracing, and nuclear microanalysis. t+ in SiO2 grown through ZrO2 was found to be ∼0.25, which means that cations and oxygen ions take part in oxide growth. As t+=0 in ZrO2 in similar conditions, the transport number is not preserved through the whole oxide thickness, indicating modification of the nature of transport events in the oxide films. Isotopic tracing experiments by the use of 30Si enriched layers on 28Si substrates show that the order of cations is preserved during plasma anodization of Si and Si through ZrO2. The consequences in terms of the microscopic mechanism of ionic transport are presented and show that processes based on point defect migration cannot explain the results; other mechanisms have to be postulated.