This study trained children to master logical fallacies and examined how learning is related to processing efficiency and fluid intelligence (gf). A total of one hundred and eighty 8- and 11-year-old children living in Cyprus were allocated to a control, a limited (LI), and a full instruction (FI) group. The LI group learned the notion of logical contradiction and the logical structure of the schemes involved. The FI group learned, additionally, to recognize other deductive reasoning principles. Reasoning improved proportionally to training. Awareness improved equally in LI and FI. Changes in reasoning and awareness changes were related to attention control and gf. Awareness mediated the influence of training on reasoning but not vice versa, suggesting that awareness is necessary for conditional reasoning. Implications are discussed.