The involvement load hypothesis (ILH), which predicts the lexical learning potential of tasks, assumes that writing sentences (SW) and compositions (CW) using novel target words (TWs) lead to similar lexical gains. However, research on the issue is scarce and contradictory. One possibility is that the higher cognitive load of CW hinders learning relative to SW. To verify the learning potential of SW and CW, we selected 20 English academic TWs and conducted a pretest-posttest quasi-experiment with Polish advanced learners of English. First, all participants wrote a control essay (without TWs), then SW participants wrote sentences and CW participants wrote two essays, each with 10 TWs. Generalized linear mixed models revealed higher gains in breadth and depth of knowledge for SW than for CW, which contradicts the predictions of the ILH. Furthermore, to detect signs of cognitive load, we derived three task-based performance measurements from the compositions: holistic scores, number of errors, and words per minute. The measurements found that the control essay and essays with TWs were of similar quality (holistic scores), but that the control essay was written faster and with fewer errors than the other two. Concluding, using TWs in essays probably increased learners' cognitive load, slowing down their writing, generating more errors, and ultimately, decreasing learning of the TWs.