A series of vulcanised blends of natural rubber (NR) with high vinyl butadiene rubber (BR) have been prepared and these exhibit polymer miscibility, as demonstrated by using a number of techniques, over a broad range of blend compositions. The susceptibility to low temperature crystallisation of sulphur cured gum and black-filled blend vulcanisates containing up to 25;high vinyl BR has been determined. In the strained state this has been determined by measuring the loss of stress associated with crystallisation and in the unstrained state by measurement of the differential scanning calorimetry melting point endotherm. A number of factors have been shown to influence the resistance of the NR blends to low temperature crystallisation. These include high vinyl content of the blend, the nature of the cure system and the presence of carbon black filler. In blend compositions containing less than 20;high vinyl BR content, little significant loss of strain crystallisation in NR occurs, as indicated by the retention of both tensile and non-relaxing fatigue properties. The use of such blends where improved resistance to low temperature crystallisation is required is discussed.