This in vitro study was to observe the effects of high temperatures on teeth restored by (1) amalgam overlaying glass ionomer bases, (2) composite/adhesive system overlaying glass ionomer bases, (3) ZnO modified temporary filling material vs. unrestored teeth. Fifty un-restored teeth (control group), 50 teeth with class I amalgam restorations and glass ionomer bases, 50 teeth with class I composite/adhesive system restorations and glass ionomer bases and 50 teeth with class I ZnO modified temporary filling material restorations were placed in a furnace and heated at a rate of 10°C/min. The effects of the predetermined 200, 400, 600, 800, 1000 and 1200°C temperatures were examined macroscopically and then observed microscopically by means of a stereomicroscope. Our observations showed that the class I restorations made of amalgam on glass ionomer bases as far as the class I restorations made of ZnO modified temporary filling material can be identified till 1200°C because they maintain their shape despite the disintegration of the crowns, whilst the class I composite/adhesive system and the underplayed glass ionomer bases remained in place in an altered shape.