Butadiene‐styrene copolymers contain the following units:trans‐1,4‐polybutadiene,cis‐1,4‐polybutadiene, 1,2‐polybutadiene, and bound styrene. Of these the first is the only type which has been shown to form crystals. By the use of published data on volume change arising from crystallization (largely due to Meyer) and on structure as a function of polymerization temperature (largely due to Hampton) it is found that crystallization in emulsion polymers has been observed only when thetrans‐1,4‐polybutadiene content has been greater than about 58 percent, becoming more complete the more this value is exceeded. From this observation one can then draw the following conclusions, all of which are in accordance with direct experimentation:(1) Crystallization is not observed if the polymerization temperature is above 60°C.(2) For polymerizations at 50°C a small amount (2 to 6 percent) of bound styrene inhibits crystallization completely.(3) For polymerizations at 5°C the limit is at about 15 to 18 percent bound styrene content.(4) At −40°C (the lowest polymerization temperature normally utilized) this limit is at about 30 percent bound styrene.