The Thinking Styles Inventories (TSI) are questionnaires for assessing individual preferences in constructing knowledge. This paper identifies several problems concerning their validity, which range from an inadequate use of factor analysis, to missing information on the measurement model, to findings indicating a low discrimination between the thinking style scales. Against this background, two studies are conducted providing detailed insights into the measurement model of the TSI in German-speaking samples (Study I: 287 apprentices; Study II: 389 students). Although results indicate a high degree of reliability according to popular statistical rules, they confirm problems with the discriminant validity and criterion validity regarding achievement. The Thinking Styles Inventories should as a result be used with caution.