During reading, the mind continuously builds and updates a discourse model—a cumulative mental world representing the gleaned information. A key operator in this process is establishing novel discourse referents—entities in the model that can be picked out. For instance, ‘She bought wool, sponges and steel’ establishes three simple referents (italicized). By contrast, ‘She bought sponges of steel wool…’ establishes two simple referents forming a composite, itself referenceable (e.g., ‘…and glued them together.’). Here, in an online reading study, we target the cognitive basis of establishing simple and composite referents in the discourse model. Participants (n=43) read 72 five-sentence English stories sentence by sentence. The fourth, critical sentence featured: (i) three simple referents (‘wool, sponges and steel’; simple3); (ii) two simple referents (‘steel wool and sponges’; simple2), or (iii) two simple referents forming a composite referent (‘sponges of steel wool’; composite). Crucially, across conditions, critical sentences had identical lengths and lexical items, while remaining sentences were fully identical. A true/false comprehension task followed each story. We hypothesized that additional referents would increase reading times (RTs) beyond syntactic, semantic, and lexical factors. Multiple regression analyses showed significant effects of adding simple (RTsimple3>RTsimple2) and composite (RTcomposite>RTsimple2) referents. The effects appeared only on critical (but not on subsequent) sentences, possibly reflecting the cognitive operators of establishing, rather than maintaining, novel referents. Our findings pave the way for future work investigating hypotheses of hierarchical structure-building in mental discourse models.