Learning-by-explaining (to fictitious others) has been shown to be an effective instructional method to support students' generative learning. In this study, we investigated differential effects of the modality of explaining (written versus oral) on students' quality of explanations and learning. Forty-eight students worked on a hypertext about combustion engines. Afterwards, they were asked to explain the learning content, either orally or in writing. Findings indicated that providing written explanations was more effective than providing oral explanations in supporting students to organize the content of the explanations. The higher levels of organization yielded higher levels of students' conceptual knowledge. In contrast, generating oral explanations, relative to written explanations, triggered students' elaborative processes to a more pronounced extent, which was more beneficial to attaining transferable knowledge. Thus, we conclude that the modality of explaining plays a critical role in learning-by-explaining inasmuch as different modes differentially support student learning.