Abstract Two specimens from a piece of stark rubber which had a melting temperature originally at 41° C were partially melted at 38° C and allowed to stand for 11 years, one at about 25° C, the other at 38° C. The first showed a continuous slow decrease in specific volume, while the second increased slowly in volume for 5 months and remained constant for about 5 months more before showing the continuous slow decrease. The melting temperature of the first increased to 45° C and the melting range was broadened. The melting temperature of the second became 52° C and the range was narrowed. Perfecting or enlarging of crystals and the effect of higher recrystallization temperatures have influenced the melting temperatures. The effect of heating rate is discussed in connection with slow increases in volume, attributed to relaxation of oriented regions, during melting.