We conducted two word-by-word reading experiments to investigate the timing of implausibility detection for recipient and instrument prepositional phrases (PPs). These PPs differ in thematic role, relative frequency, and possibly in argument status. The results showed a difference in the timing of garden path effects such that the detection of implausible dative recipients (which are clearly arguments) was delayed relative to the detection of implausible instruments (which may not be arguments). They also demonstrated that commitments to syntactic structure were made at the preposition for both dative and instrument PPs. While these results refute delay models of parsing (e.g., Britt, 1994) and syntax-first accounts of PP-attachment (e.g., Frazier, 1978; Frazier & Clifton, 1996), they support constraint-based lexicalist models that enable verb bias and plausibility information to compete (Garnsey, Pearlmutter, Myers, & Lotocky, 1997).