The French Museum of Natural History (MNHN, Paris) hosts one of the major collection of aphids of the world. Built up from 1950 by the entomologist Georges Remaudière, and composed of numerous microscope slides associated to more than 140 publications and new taxa, this collection demonstrates Remaudière‘s contribution to the field of entomology and aphids taxonomy. From 1950 to the 90’s, specimens were mounted in gum-chloral mounting media, before using Canada balsam. Nowadays, most of the gum-chloral mounting media display a color change, ranging from a slight to an extensive darkening (depending on the slides) which could be damaging for the study of the specimens, including types. As this phenomenon touches other collections worldwide, it is important to try and identify the chemical reactions at stake and to understand the parameters affecting the degradation in order to improve the preservation of the microscopic slides. Our research was carried out in three main stages: i) a partial inventory of the collection to assess the state of conservation and cross-check information about date of specimen collection ii) the identification of the gum-chloral recipe(s) used in the collection; and iii) the identification of molecules potentially responsible for the darkening. To identify the recipes involved in the collection and molecules behind darkening, chemical analyses such as infrared spectroscopy (ATR-FTIR) and gas chromatography (GC-MS) were used. Historical mounting media (40 years old) and slide’s mounting media were analyzed and compared with modern recipes as references. A specific recipe of gum-chloral mounting media proved to be problematic and some of the molecules responsible for the coloration were identified. Hypotheses will be presented on the parameters causing the deterioration, which are still being studied.