This contribution will examine ethical questions in cataloguing Arabic manuscripts in private collections in the Middle East, based in 10 years of experience in cataloguing private libraries of ‘ulama’ in Yemen (Zabid, on theRed Sea). As this paper explores, in the Arabian Peninsula there exist various religious pressures when it comes to buying, cataloguing or studying manuscripts, due to their content. Sometimes they are unspoken, other times they are explicit. Access to private libraries by foreigners can be limited until a trusted relationship is established, though for locals it often is totally denied for ‘ethical’ reasons, among them the desire to regulate access to knowledge, and social evaluations of the status of owners. Once inside the library, particular points of misunderstanding or tension are related to different ways local and foreign scholars classify types of books. Even the difference between manuscripts and printed books can be a point of disagreement, but this contribution will focus on the status of Qur’ans and books on occult sciences (al-‘ulum al-khafiyya). Qur’ans are a complicated question, as can be observed in different catalogues in Arabic where sometimes they appear to be books as the others and sometimes not, an issue I will also address from the point of view of local practices. Books on occult sciences are another special category of items surrounded by difficulties, where owners of the libraries are concerned to prevent the unprepared (and unprotected) reader from bringing harm upon themselves or others through exposure to what is in these books—with catalogers considered to belong this group of unprepared readers. These experiences raise numerous ethical questions for the cataloger. We can abandon our efforts, or attempt to anticipate such pressures, but if we are keen to follow them then we can be tempted not to catalogue all the books in a private library, i.e. to make a selection. From the point of view of a scientific ethic (e.g. codicological and conservational), everything that is on the shelves of a private library has to be catalogued without a single exception, including photocopies of manuscripts. Being exhaustive will allow us to have a comprehensive idea of the content of a library of a ‘alim—or that of another layer of the society—and also to map endangered collections. However, one must also take into consideration the views and concerns of owners. It is the balance between these things that this paper addresses.