In this article, data from the Reading Literacy study conducted in 2000 and 2001 as part of the OECD "Programme for International Student Assessment" (PISA) were analysed in order to explore equivalence issues across the 47 test adaptations in various languages of instruction that were used by the participating countries. On average, about 82% of the variance in relative item difficulty were found to be common across the various national versions. However, the index of equivalence appeared to be lower than desirable in certain categories of countries. Tentative analyses were conducted to better understand the reasons behind these differences.