: The use of gypsum mortars for interior plastering of walls and ceilings is almost as old as building construction for human dwelling purposes. In Portugal, in spite of the lack of raw material of good quality, the archaeologists' community, as well as other authors, thinks it could have been used since the Roman period (Silva [1]). However, its presence was particularly expressive in the Portuguese architecture between the 18th century and the first half of the 20th century, in the shape of gradually simplified decorative programs. The quality and variety of gypsum plaster decorations in the Portuguese monumental and domestic architecture, contrasts with the rare studies on this subject, from the material point of view. Furthermore, information about this heritage is almost inexistent, which is leading to a fast loss of important heritage. In this paper, the results of a simple screening XRD test to detect the presence of gypsum in the samples already collected are reported, as well as the complete XRD, TGA-DTA and optical microscopy observations carried out for the characterization of those which revealed to be gypsum plaster samples. The present research occurs in this context, where a chemical, physical and micro-structural characterization of the Portuguese historical gypsum mortars is the first step.