The work reports the results of measurements of acoustical parameters obtained in six abbeys of the Cistercian order (XIIth century). Three of them, Silvacane, Senanque and Le Thoronet, are in the South of France. They have the original layout both outside and inside the building, and are finished in local stone. The values of the reverberation time RT, measured in these abbeys, are higher than in other religious buildings of almost the same volume. The other three abbeys, at Tiglieto, Morimondo and Chiaravalle, in Northern Italy, have similar characteristics as regards the plan and the simplicity of the interiors, but their inner surfaces are of bricks and stones, in different proportions. The measured RT values are lower than those of the French abbeys. In the abbey at Le Thoronet, the RT values are higher at low and middle frequencies, which corresponds to the range of liturgical and mainly Gregorian choral music, played in Cistercian abbeys without musical instruments. This effect could have been the aim of the Cistercian architects, experienced in architectural acoustics, or is the consequence of the use of a special kind of stone, that has lower absorbing properties than the materials used in the other churches.