This paper discusses a study that explored the mental models of fifth (n=35) and sixth (n=49) graders attending two urban public elementary schools in South Korea in word problem solving. Two levels of mental models (problem model and mathematical model) constructed in the process of word problem solving were investigated. Coding categories for both problem and mathematical models were developed to be used in think-aloud protocol analysis. Results indicate that the majority of students who constructed correct problem models rapidly classified the problems as they read them or restated the problems, focusing on specific words, and related these words to other statements in the problems. Students who misrepresented the problems varied in their problem models: incorrect relation of specific words to other information model, diagrammatic-incorrect location of information model, number-consideration model, and number-operator model. Performance levels in problem solving are discussed in relation to the problem model and mathematical model. Includes samples of solutions. The appendices contain the categories and sample behavior used to determine problem models and the categories used to determine mathematical models. Includes 36 references. (Author/MKR)