In addition to information about phonology, morphology and syntax, lexical entries contain semantic information about participants (e.g., Agent). However, the traditional criteria for determining how much participant information is lexically encoded have proved unreliable. We have proposed two semantic criteria (obligatoriness and selectivity) that jointly identify the participants that are lexically encoded in verbs. We tested whether one of these criteria, semantic selectivity, makes psychologically real distinctions between participant information that is lexically encoded and participant information that is not. We examined how readers integrated syntactically optional WH-constituents in filler-gap sentences when the participant information conveyed by the WH-filler was specific to a restricted class of verbs (i.e., source locations) and when it was not (i.e., event locations). Our results provide support for the role of specificity in the lexical encoding of participant information of syntactically optional constituents.