The water quality of rivers and lakes can be determined among others by using diatoms, single-celled microscopic algae, which is a mandatory part of the Water Framework Directive in Europe. Comparison with the previous status of the water bodies can be done by using previously taken samples at the same location for the study of these organisms. In many tropical areas, however, such samples are not available. Here, herbarium material can possibly provide a solution as certain diatom species live attached to aquatic plants. These diatoms remain present when aquatic plants are collected, dried, mounted and classified in the herbarium. However, studies of diatoms on herbarium specimens are scarce. For tropical Africa, for instance, only two publications can be mentioned. We explore what possibilities, apart from determining past water quality, the investigation of diatoms present on herbarium material from tropical Africa can offer, such as the study of biodiversity. Finally, the advantages and disadvantages of using epiphytic diatom studies on herbarium material of aquatic macrophytes sampled in the Democratic Republic of the Congo for past water quality studies are discussed.