This investigation studied the effect of thermocycling on the hardness of six commercially available soft lining materials. The apparatus used to measure hardness measured the difference between depth of penetration of an indentor into a specimen under a primary contacting load and a secondary penetrating load. Control specimens were stored in water at 20 ± 2 degrees C and trial specimens were thermocycled between 18 =- 1 degree C and 53 =- 1 degree C for periods up to 28 days. It was found that thermocycling had a deleterious effect on the softness of all six materials. Further, it was shown that chairside provisional soft lining materials demonstrate a rapid deterioration at temperatures greater than 50 degrees C.