There is increasing evidence that skilled readers of English benefit from processing a parafoveal preview of a semantically related word. However, in previous investigations of semantic preview benefit using the gaze-contingent boundary paradigm the semantic relatedness between the preview and target has been confounded with the plausibility of the preview word in the sentence. In the present study, preview relatedness and plausibility were independently manipulated in neutral sentences read by a large sample of skilled adult readers. Participants were assessed on measures of reading and spelling ability to identify possible sources of individual differences in preview effects. The results showed that readers benefited from a preview of a plausible word, regardless of the semantic relatedness of the preview and the target. However, there was limited evidence of a semantic relatedness benefit when the plausibility of the preview was controlled. The plausibility preview benefit was strongest for low proficiency readers, suggesting that poorer readers were more likely to program a forward saccade based on information extracted from the preview. High proficiency readers showed equivalent disruption from all nonidentical previews suggesting that they were more likely to suffer interference from the orthographic mismatch between preview and target.