If a Mandarin speaker had walked past two rivers and wished to describe how many he had seen, he would have to say “two tiao river”, where tiao designates long, rope-like objects such as rivers, snakes and legs. Tiao is one of several hundred classifiers – a grammatical category in Mandarin. In two eye-tracking studies we presented Mandarin speakers with simple Mandarin sentences through headphones while monitoring their eye-movements to objects presented on a computer monitor. The crucial question is what participants look at while listening to a pre-specified target noun. If classifier categories influence general conceptual processing then on hearing the target noun participants should look at objects that are also members of the same classifier category – even when the classifier is not explicitly present. For example, on hearing scissors, Mandarin speakers should look more at a picture of a chair than at an unrelated object because scissors and chair share the classifier ba. This would be consistent with a Strong Whorfian position, according to which language is a major determinant in shaping conceptual thought (Sapir, 1921; Whorf, 1956). A weaker influence of language-on-thought could be predicted, where language shapes cognitive processing, but only when the language-specific category is actively being processed (Slobin, 1996). According to this account, eye-movements are not necessarily drawn to chair when a participant hears scissors, but they would be on hearing ba scissors. This is because hearing ba activates the linguistic category that both scissors and chair belong to. A third logical possibility is that classifiers are purely formal markers (cf. Greenberg, 1972; Lehman, 1979) that do not influence attentional processing even when they are explicitly present. The data showed that when participants heard a spoken word from the same classifier category as a visually depicted object (e.g. scissors-chair), but the classifier was not explicitly presented in the speech, overt attention to classifier-match objects (e.g. chair) and distractor objects did not differ (Experiment 1). But when the classifier was explicitly presented (e.g. ba, Experiment 2), participants shifted overt attention significantly more to classifier-match objects (e.g. chair) than to distractors. These data are incompatible with the Strong Whorfian hypothesis. Instead the findings support the Weak Whorfian hypothesis that linguistic distinctions force attention to properties of the world but only during active linguistic processing of that distinction (cf. Slobin, 1996).