Brain and Language 68, 205–211 (1999) Article ID brln.1999.2081, available online at http://www.idealibrary.com on The Mass/Count Distinction: Evidence from On-Line Psycholinguistic Performance Brendan Gillon McGill University, Montreal, Quebec, Canada Eva Kehayia McGill University and Jewish Rehabilitation Hospital, Montreal, Quebec, Canada and Vanessa Taler McGill University, Montreal, Quebec, Canada Under the hypothesis that the mass/count distinction in English is marked by a monovalent lexical feature, this article investigates whether features, lexical or morphosyntactic, play a role in simple lexical decision. Research findings have yet to settle how many features are accessed during lexical decision and to what extent morphosyntactic features are computed out of context. We used two on-line lexical decision experiments (simple and morphosyntactic priming). Results show that the lexical feature ‘‘mass’’ is computed in both experiments. However, the morphosyn- tactic feature ‘‘plural’’ is subject to task-specific effects and surfaces only where operative.  1999 Academic Press Key Words: mass; count; singular; plural; feature; lexical access; lexical semantics. Psycholinguistic investigations of the mental lexicon have, thus far, shown that an increased processing load, either of grammatical features encoded in the representation or of morphological operations, can affect reaction times (RTs) during word recognition in on-line psycholinguistic experiments (Taft & Forster, 1975; Laudanna et al., 1992; Niemi et al., 1994; Kehayia & Jarema, 1994). Little is known about the role of grammatical features in on- line word recognition; even less is known about such features as mass for nouns, though neurolinguistic findings have shown dissociations in the per- formance of patients on different noun classes, including mass and count nouns. Address correspondence and reprint requests to Brendan Gillon, Department of Linguistics, McGill University, 1085 Avenue Docteur-Penfield. Montreal, Quebec H3A 1A7, Canada. 205 0093-934X/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.