This research examined the degree of comparability and sources of incomparability of English and French versions of reading, mathematics, and science tests that were administered as part of a survey of achievement in Canada. The results point to substantial psychometric differences between the 2 language versions. Approximately 18% to 36% of the items were identified as differentially functioning for the 2 language groups. Large proportions of these differential item functioning (DIF) items, 36% to 100% across age groups and content areas, were attributed to adaptation related differences. A smaller proportion, 27% to 33% of the DIF items, was attributed to curricular differences. Twenty-four to 49% of DIF items could not be attributed to either of the 2 sources considered in the study.