Differential item functioning (DIF) can undermine the validity of cross-lingual comparisons. While a lot of efficient statistics for detecting DIF are available, few general findings have been found to explain DIF results. The objective of the article was to study DIF sources by using a mixed method design. The design involves a quantitative phase in which DIF was analyzed followed by a qualitative phase conducting cognitive interviews. To illustrate the proposal, polytomous DIF was analyzed in the scales from the PISA (Programme for International Student Assessment) Student Questionnaire (Organisation for Economic Co-operation and Development). Evidence obtained allowed DIF to be connected with differences in the interpretation patterns of participants from the different linguistic groups. Finally, benefits of mixed methods design for analyzing equivalence in cross-lingual assessments are discussed.