Second language vocabulary acquisition has been modeled both as multidimensional in nature and as a continuum wherein the learner's knowledge of a word develops along a cline from recognition through production. In order to empirically examine and compare these models, the authors assess the degree to which the Vocabulary Knowledge Scale (VKS; Paribakht & Wesche, 1993), which implicitly assumes a cline model of acquisition, conforms to a linear trait model under the Rasch Partial Credit Model, and determine the dimensionality of the individual tasks contained on the scale (selfreport, first language [L1] equivalent, and sentence) using DETECT. The authors find that, although the VKS functions adequately overall as a measurement model, Stages 3 (can give an adequate L1 equivalent) and 4 (can use with semantic appropriateness) are psychometrically indistinct, suggesting they should be collapsed into a single category of definitional knowledge. Analysis under DIMTEST and DETECT indicates that other forms of vocabulary knowledge measured by the VKS are weakly multidimensional, which has implications for continuum models of vocabulary acquisition.1. The word is not familiar at all. 2. The word is familiar but its meaning is not known. 3. A correct synonym or translation is given. 4. The word is used with semantic appropriateness in a sentence.