The effects of the number of fat layers (4-162 layers) on expansion and loss of CO2 and water in Danish pastries (yeasted puff pastry) were studied at each processing step. Proving contributed to most of the expansion (200-300%) and no specific role of fat layers was evidenced at this stage; this was confirmed by measurements of CO2 released. Relative expansion during baking ranged from 20% to 65% for 4 to 32 fat layers and remained unchanged above 48 layers. Images cross sections of Danish pastry showed that bubbles were few for 4 fat layers, but very large (3-4 cm) and elongated in the direction of layering. For high numbers of fat layers, bubbles were numerous, smaller and round (2-7 mm). The loss of water vapor retention during baking in relation to fragmentation of fat layers during sheeting explained the leveling-off of expansion and the decreasing size of bubbles. © 2015 Published by Elsevier Ltd.