This article describes the construction of a hierarchical taxonomy of metacognitive activities for the interpretation of thinking-aloud protocols of students in secondary education, who studied texts on history and physics. After testing an initial elaborate taxonomy on a restricted number of protocols by multiple raters, it appeared that the interrater correspondence was well below par. The categories in the taxonomy were too highly specified. Categories were combined and tested on new protocols in a cyclic fashion. The revised taxonomy was then used for coding 16 history protocols and 16 physics protocols. Frequencies of occurrence of metacognitive activities were obtained, as well as judgements of the quality of the metacognitive activities of the participants. There is a reasonable correlation between the frequency method and the quality method for coding thinking-aloud protocols. Also, there is a substantial covariation of the number of metacognitive activities across both tasks. (Contains 2 tables and 6 figures.)