This study investigates whether the perceptual salience of grammatical morphemes influences the online processing of temporarily ambiguous sentences during adult first-language (L1) comprehension. In a bidirectional self-paced reading task, adult L1 English participants (N = 44) read sentences with time adjuncts that were in a structural position in which they could attach either to the most recent verb phrase (VP) or to a VP in a higher clause. Consistent with previous findings, the reading times on these sentences indicated processing difficulty when this adjunct allowed only for high attachment. Crucially, this effect was modulated by the perceptual salience of the grammatical morphemes used to indicate time reference in these clauses. Specifically, the processing cost for high attachment was larger when time in the lower clause was indicated by the auxiliary verb will compared to when it was indicated by the relatively less salient past -ed morpheme. These findings were taken to indicate that the influence of perceptual salience extends beyond the acquisition of and sensitivity to grammatical morphemes during L1 and L2 development. Rather, the perceptual salience of these forms also appears to affect online structural processing during adult L1 sentence comprehension.