This study examined how engaging calculus students in Writing to Learn Mathematics affected the types of conceptual and procedural errors that the students made on their examinations. Students in two sections of an introductory college calculus course in Fall 1994 were the respondents in this study. A classification system was developed that categorized students' errors as procedural, conceptual, or indeterminate. Procedural errors involved either syntactical or algorithmic errors. Conceptual errors involved use of inappropriate procedures, acceptance of unreasonable answers, translation mistakes, misuse of symbols, incorrect interpretation of symbols, invalid inferences, statements without justification, or contradictions of nonprocedural principles, definitions, or theorems. (Author/SW)