Schubert challenged the high Classical style’s implied boundaries by gradually incorporating non-normative procedures into his sonata-form practice and, more specifically, into his treatment of transitional spaces. Among these non-normative procedures are his tonally overdetermined transitions: transitions that struggle to leave the tonic area, often introducing formal and expressive complications to the work’s unfolding. This paper examines the impact of tonally overdetermined TRs on the MC in Schubert’s sonata forms, demonstrating how TR’s penchant for the tonic area may ultimately define the MC’s formal and expressive roles. It adopts Hepokoski and Darcy’s Sonata Theory as a theoretical framework and is organized in three categories that are defined by the position, function, and strength of the I:PACs articulated (or proposed only) within pre-MC space. The first category involves transitions that fail to leave the tonic, ending in a I:PAC MC. In the second, a I:PAC is followed by a “defective” passage that can only be retrospectively reinterpreted as TR after the articulation of the MC. The third category considers transitions that begin with an extended tonic prolongation and end with a quick and abrupt modulation. The conclusion shows that the formal and expressive effects released by Schubert’s tonally overdetermined TRs extend well beyond their realization and, in most cases, involve the MC as protagonist.