Marsh's (1986) Internal/External Frame of Reference (I/E) model posits a paradoxical pattern of relations between distinct school subjects, for example, math and verbal. (1) A high math achievement contributes to a positive math self-concept, and a high verbal achievement contributes to a positive verbal self-concept. (2) A high math achievement contributes to a negative verbal self-concept, and a high verbal achievement contributes to a negative math self-concept. An extension of the I/E model is proposed under which a comparison is made of the relationship between math and science, which might be regarded as distinctly different or as supplementary, and which process is termed "domain comparison". In addition, the "supplementary" perspective (or integrating process) is a deviation from the "distinctly different" perspective (or contrasting process) that has long underpinned the I/E model. The results of exploring the I/E model for math and science across 28 countries (N = 144,069) participating in the TIMSS 2003 study revealed students from different countries undertake a contrasting or an integrating process to varying degrees. The findings suggest an incorporation of domain comparison into the I/E model. (Contains 4 tables and 1 figure.)