This study attempts to estimate Structure of Learning Outcome (SOLO) levels in mathematics using the Partial Credit and Rating Scale models. A 30-item test comprising 10 testlets of 3 items each was designed and administered to 674 lower secondary school students. The items were arranged in a hierarchical manner, each testing SOLO levels in this order: Unistructural, Multistructural, and Relationship/Abstract. The item response matrix was fitted into the Partial Credit and Rating Scale models. Results showed that: (1) the observed testlet response patterns fitted those expected; (2) the dataset fitted the psychometric models reasonably well; and (3) the proportion of examinees getting SOLO items correct decreased in order from level 1 to level 3 along the math proficiency continuum between -2.0 and +2.0. The results of the study have implications for a criterion-based approach in interpreting test results based on SOLO testlets. Results also showed the viability of testlet item bank development, test construction, and computerized testing using testlets. (Contains 1 figure, 4 tables, and 20 references.) (Author/SLD)