FTIR handheld devices, working in the diffuse reflectance mode (DRIFT), are promising analytical instruments to perform in situ analyses on cultural heritage materials. However, in the analyses performed in situ with such DRIFT handheld devices, distortions in the DRIFT spectra can be observed in the measurements due to the presence of specular reflection, showing inverted bands in those IR bands with the highest absorption index. These distortions present in the spectra obtained with handheld devices make their resolution very difficult unless the working mode of the devices is well known and a suitable DRIFT database is available. With the aim of getting the most suitable tools to perform analyses in the field, this work has been developed considering two important aspects. In the first one, the differences between the spectra obtained in transmittance, attenuated total reflectance and diffuse reflectance modes of some compounds (nitrates, sulphates and carbonates) that could show the inverted bands when measured with a FTIR handheld device have been studied. In the second one, a preliminary database of several compounds that can be found as original (bulk) compounds or as efflorescence in affected built heritage materials, obtained in the laboratory in diffuse reflectance mode, is presented. Finally, the usefulness of the presented database has been tested using the spectra obtained in situ in the Fishermen's association building (San Sebastian, Basque Country, north of Spain) on areas presenting several decaying processes.