This article reports an empirical study investigating L2 acquisition of the Mandarin Chinese collective marker -men by adult Thai-speaking learners and the Thai collective marker phûak- by adult Chinese-speaking learners within the framework of the Feature Reassembly Hypothesis (Lardiere, 2009a, 2009b). An acceptability judgment test was administered to learners with beginning, intermediate and advanced proficiencies of Chinese and Thai (n = 114) as well as native speaker controls (n = 30). The results reveal a facilitating role of positive evidence in L2 feature reassembly as Chinese learners who are exposed to positive evidence of “phûak + animal noun” and “phûak + indefinite noun” structures in their Thai input perform native-like on these structures from an intermediate level onward. On the other hand, feature reassembly is hindered when positive evidence is unavailable as in the case of Thai learners of Chinese where no evidence they receive in the input shows ungrammaticality of “animal noun + men” and “indefinite noun + men” structures in Chinese. These learners mostly fail to perform native-like even at an advanced level.