Source density analysis of EEG recordings from 12 healthy human volunteers was used in a randomized, placebo controlled cross over study to investigate the change in physiological parameters after ingestion of a soft drink containing green tea extract enriched with L-theanine and theogallin. EEG was recorded 1, 2, 3 and 4 h after ingestion during different recording conditions. Visually evoked P300 potentials were recorded every hour in addition to the EEG recordings. Analysis of the data revealed a general attenuation of electrical delta power under the condition of eyes open during the first hour (statistically significant at p < 0.01). During a reading test increases of delta and theta power were observed at frontal electrode sites starting with the second hour after administration, significant at the third and fourth hour (p < 0.04) in comparison to placebo. These changes indicate a higher level of mental performance. Increases of beta 1 power starting with the second hour indicated a higher degree of relaxation. However, no statistical significance was reached. Analysis of visually evoked P300 waves revealed a decrease in latency at the last hour (statistical significance p < 0.04) as well as increases of amplitudes at the electrode position Cz (from the first to the third hour, statistically not significant). This type of result in general suggests an improvement of attention. Thus, decaffeinated extract of green tea still has a stimulating effect despite the lack of caffeine presumably due to its high content in L-theanine and theogallin as found in preclinical experiments.