Transcranial direct current stimulation (tDCS) is a promising, emerging technique for the treatment of an array of neurological and psychiatric disorders but little is known about the mechanism(s) by which it might work. We used 31P magnetic resonance spectroscopy to measure bioenergetics effects of applying 1 mA of tDCS for 10 min in 13 healthy volunteers . Brain bioenergetics were measured from the left temporo-frontal region using 31P magnetic resonance spectroscopy before, during and for 20 min following tDCS. Brain pH rose during tDCS and remained elevated afterwards. Phosphomonoesters were significantly decreased while inorganic phosphate (Pi) also fell. Partial-least squares discriminant analysis of the data revealed two significantly different subject groups: one where phosphocreatine (PCr), ATP and Pi fell along with a larger increase in pH and one where PCr and ATP increased along with a smaller increase in pH and a slower and more sustained decrease in Pi. Group membership was predicted by baseline pH and ATP. We interpreted the effects of tDCS as driving two biochemical processes: cellular consumption of ATP causing hydrolysis of PCr via the creatine kinase reaction driving the increase in pH; synthesis of ATP and PCr by mitochondria with concomitant drop in Pi and phosphomonoester levels. Additionally, we have used time series intervention analysis and T2* imaging to map the effects of tDCS . Whilst initially promising, this mapping approach is being replaced with new system identification methods.