The Saint Joseph chapel in the city of Caluire et Cuire, France is an historic monument built in 1885 by the architect Sainte Marie Perrin. CSTB has been asked to study the acoustics of the chapel. The aim is to create an acoustic environment that permits non-amplified performances such as chamber music with the goal of a reverberation time of 1.5s. Characterizing acoustic measurements have been done in the chapel. The measurement results are presented. They show a reverberant acoustic which is too high for classical music. After analysis of the acoustic measurements, CSTB was asked to propose modifications to improve the acoustics. These proposed modifications have to take into account the constraints due to the historical shape and architecture of the chapel. Two different solutions are proposed. The first solution is based on greatly increasing the absorbing surfaces and comes close to the ideal acoustic for classical music. The second solution allows the improvement of the actual acoustic with less absorbing material. Some diffuse and mode breaking surfaces are added. This second solution also includes a proposal for adapted music in this architectural and acoustical framework.