AbstractOutdoor and indoor pollutants can affect the conservation of natural history collections. As a first approach to this question, environmental acidity around specimens in the collections of the Natural Science Museum of Barcelona and Botanic Institute of Barcelona (MCNB‐IBB) was measured. Measurements were taken using optical sensors in a number of different places: inside and outside buildings, in storerooms, on store cases and cabinets and in containers in three collections. All acidity measurements fell within a pH range of 6.5–7.7, which suggests that the collections tested were not exposed to a dangerous acidic environment during the measurement period. Despite its outdated facilities, the Museum’s buildings and storerooms act as effective barriers against acid pollutants entering from the outside. The type of storage facilities and containers employed seems to have little impact but is still nevertheless of some relevance. Optical sensors are excellent tools for disentangling the complex spatio‐temporal environments in which the MCNB‐IBB collections are housed. Larger samples will be necessary if a more complete analysis is to be undertaken.