Uittenhove and Lemaire (Exp Psychol 59(5):295-301, 2012) found that we are slower when executing a strategy following a difficult strategy than when executing the same strategy following an easier strategy (i.e., strategy sequential difficulty effects). Uittenhove and Lemaire suggested that difficult strategies temporarily reduce available executive capacities, interfering with the next strategy execution. In this study, we used ERP to determine the time course of these effects. In a computational estimation task, we found greater cerebral activities during strategy execution following a more difficult compared to an easier strategy. Interestingly, greater cerebral activities were most apparent immediately after the encoding of the problem and not during encoding or in later stages of processing. This suggests that strategy sequential difficulty effects interfere most with the retrieval of procedures in contrast to execution of these procedures. We discuss implications of these findings for further understanding of execution of cognitive strategies.