Abstract In the process of making a final impression for complete dentures, the dimensional stability of the impression tray is an important factor. The aim of this study was to measure the dimensional stability of trays of different materials, namely shellac (SH), clear thermoplastic acrylic (TP) and self-curing acrylic (SC), during the process of border moulding. Changes were measured in upper and lower trays immediately after manufacture, following storage for 24 h, directly following border moulding and 1, 2 and 14 days after moulding. The trays were manufactured on a metal master model on which nine reference points for the upper tray and eight for the lower were measured by means of a measuring microscope. The dimensional changes relative to the master model were calculated. Self-curing acrylic and thermoplastic trays proved to be stable during the process of border moulding. The lower impression trays of shellac suffered appreciable shrinkage and were considered unreliable.