Neuroscientific research on human intelligence yielded evidence that brains of highly intelligent individuals work more efficient in a way, that they display more focused and therefore overall less cortical activation while working on cognitive tasks (Haier et al.., 1992, for an overview see Neubauer & Fink, 2009). Furthermore, Doppelmayr et al.. (2005) could show that task difficulty moderates cortical activation. Persons of higher intelligence exhibited more brain activation during difficult tasks. A concept that might account for this finding is mental effort. This concept can be understood as the voluntarily mobilisation of mental energy or resources of attention towards a task, which could be interpreted as a compensatory strategy (Hockey, 1997) Therefore, the present study was conducted in order to investigate the influence of task difficulty, intelligence and achievement motivation on neural efficiency. N = 53 high school students worked on number series, which were administered according to their personal ability level, in three different conditions (easy vs. medium vs. hard). High-intelligent persons showed similar cortical brain activation (event-related desynchronisation, ERD) than low-intelligent persons in the upper alpha band, when items were delivered/shown according to their ability level. A four-way interaction of ipsative item difficulty level x IQ group x achievement group x area emerged. Post-hoc analyses showed that at anteriofrontal (AF) sites less ERD could only be observed for the IQ-low, avoidance subjects compared to IQ-low, approach-driven and IQ-high, avoidance-driven subjects within the easy condition. The obtained results are discussed in terms of expectations and recent literature on motivation and mental effort served to lay the ground work for explaining the results.