Upon hearing a novel label, listeners tend to assume that it refers to a novel, rather than a familiar object. While this disambiguation or mutual exclusivity (ME) effect has been robustly shown across development, it is unclear what it involves. Do listeners use their pragmatic and lexical knowledge to exclude the familiar object and thus select the novel one? Or is the effect, at least in early childhood, simply based on an attraction to novelty and a direct mapping of the novel label to a novel object? In a preregistered online study with 2- to 3-year-olds (n = 75) and adults (n = 112), we examined (a) whether relative object novelty alone (without pragmatic or lexical information) could account for participants' disambiguation and (b) whether participants' decision processes involved reasoning by exclusion. Participants encountered either a known and an unknown object (classic ME condition) or two unknown objects, one completely novel and one preexposed (novelty condition) as potential referents of a novel label. Reasoning by exclusion was assessed by children's looking patterns and adults' explanations. In the classic ME condition, children and adults significantly chose the novel object and both used reasoning by exclusion. In contrast, in the novelty condition, children and adults chose randomly. Across conditions, a retention test revealed that adults remembered their prior selections, while children's performance was fragile. These results suggest that referent disambiguation is not based on relative object novelty alone. Instead, to resolve referential ambiguity, both young children and adults seem to make use of pragmatic and/or lexical sources of information and to engage in reasoning by exclusion strategies.