The French Museum of Natural History (MNHN, Paris) hosts the valued collection of Georges Deflandre, algologist and father of modern micropaleontology. Built up from1920, and composed of about 13 000 microscope slides associated with numerous publications, this collection testifies of Deflandre‘s contribution to the field of micropaleontology. Those slides contain an impressive diversity of mounted specimens (diatoms, radiolarians, flagellates...) in various mounting media (glycerin jelly, synthetic or natural resins...). Nowadays, most of the media display a coloration (yellow, brown), cracks or dryness, which could be damaging for the study (and possibly the exhibition) of the specimens, including types. The objective of the project was to inventory the techniques used by Deflandre (mounting medium, sealant, dyes) and the state of preservation of the slides (mounting medium, sealant, cover glass, slide, labels) to assess the durability of the preparations. First, the recipes and mounting techniques were documented using the available literature. Then, an inventory was done on approximately 900 slides in the collection using a spreadsheet with pre-defined entries, designed to facilitate analysis on the collected information. Well documented labels and some chemical analysis were used to fill in the fields for the mounting media and dyes. The use of more than fifty combination of dyes, nineteen mounting media and four sealants was revealed. Three aspects regarding the media state were particularly documented as they affect the slide visibility or the long-term conservation: the color (on the range from yellow to brown), the mechanical degradation (cracks, breaks) due to ageing and the presence of air related to a defective preparation (bubbles, drying). The possible link between yellowing and mechanical degradation was evaluated, as well as the impact of the sealant degradation on the drying of the mounting medium. The cross-referencing of information between literature and slides allowed us to better understand the mounting practices. Additionally, observing these 50 to 100 years old slides was one of the best ways to assess the conservation of mounting media trough time. The methodology developed in this work could be applied to other slides collections.