We offer a re-evaluation of the tone-monitoring technique in the study of parsing. Experiment 1 shows that reaction times (RTs) to tones are affected by two factors: a) processing load, resulting in a tendency for RTs to decrease across a sentence, and b) a perceptual effect which adds to this tendency and moreover helps neutralise differences between sentence types. Experiment 2 successfully discriminates these two factors by registering event-related brain potentials (ERPs) during a monitoring task, establishing that the amplitudes of the N1 and P3 components —the first associated with temporal uncertainty, the second with processing load— correlate with RTs. Experiment 3 then behaviourally segregates the two factors by placing the last tone at the end of sentences, activating a wrap-up operation and thereby both disrupting the decreasing tendency and highlighting structural factors.