Felix Mendelssohn’s Lieder ohne Worte usually imply the three-part ‘song form’ through their generic title and formal layout. In many of those cases, however, the allegedly simple song form constitutes a surface layer that conceals rather complex formal procedures. In this paper, I will focus on one prototype of such formal ambiguity: the expansion of a ternary or binary form by means of procedures specifically associated with the sonata form (“sonaticiztion”). In my paper I will present formal analyses of three case studies—the op. 19 no. 5, op. 30 no. 4, and op. 85 no. 3—and show how processes and transformations that reflect the formal and generic framework of the sonata form resonate within Mendelssohn’s piano lieder. I will point at three principle procedures associated with sonata form: (1) the expansion of a developmental middle section, which becomes equally long or longer than the outer parts; (2) the inclusion of a second-theme—or a second-theme-like unit—that is presented in a non-tonic key and returns in the tonic region; and (3) the emphatic distinction between sections that are more “tightly-knit” and comparatively “loose” developmental units (BaileyShea 2002). In conclusion I will address the generic ambivalence of Mendelssohn’s piano lieder. While these works seem to follow the ideology of Romantic fragment and its inclination toward enigmatic condensation (Perrey 2002), they concurrently encapsulate the self-contained quality of the sonata form and the classical ideals (Dahlhaus 1974, Rothstein 1989, and Botstein 2001). Viewed from this dialectical lens, the Lieder ohne Worte emphasize Mendelssohn’s pivotal role in the development of Romantic musical forms (Taylor 2011) and represent a unique instance of the tangled interaction between Classical and Romantic styles and aesthetics.