This study compared the thickness of powder layers sprayed on Cerec inlay cavity preparations before taking the "optical impression". 6 different powder propellant methods were evaluated. 60 MOD inlay cavities were prepared in vitro in extracted human molars and mounted in phantom-head jaws. 10 clinicians powdered 1 cavity each for each of the 6 methods. The powdered teeth were embedded in acrylic and the preparations sectioned parallel to the cavity floor, mesio-distally and bucco-lingually. The powder layer thickness on the proximo-lateral walls (PLW), cavity floors (CF) and occlusal margins (OM) were measured using a light microscope. The means and SD are: [Table; see text] This study showed that propane-butane propellants were as effective as the fluoridated hydrocarbons. A wet system (5) was good despite the transient surface cooling (-43 degrees C) effect. However, the air-flow system (6) produced acceptable powder layers as well, avoiding any chemical contamination of the room air.