Findings on executive function (EF) training gains remain inconsistent, with varied outcomes among participants. While factors such as age, baseline cognitive abilities, and personality traits are commonly explored as predictors of EF training gains, limited attention has been given to the neurophysiological markers associated with training gains. This line of research could be potentially fruitful, as theta and alpha band power have been associated with performance on various cognitive measures. In our study, we aimed to address this gap by examining whether relative theta and alpha power can predict gains in updating and inhibition training beyond baseline cognitive performance. A group of 20 healthy middle-aged adults (49-65 years) were randomly assigned to either an updating or inhibition training group, participating in twenty 20-minute sessions over ten weeks. Resting-state EEG data were recorded before the first training session. Mixed-effects model analyses revealed that participants with higher scores on fluid reasoning tasks showed higher training gains. However, relative theta and alpha power did not emerge as additional predictors of training gains in either the updating or inhibition training groups. Considering individual differences in EEG power bands, further investigation is needed with individually defined power bands.