The heats of solution in orthodichlorobenzene were measured as a function of temperature for both as‐reprecipitated and quenched atactic polymethylmethacrylate. The as‐reprecipitated curve decreases linearly with a slope of 0.096 cal/g°C from room temperature to 90°C, and above 110°C has a constant value of +1.5 cal/g. The quenched material data decreases linearly with a slope of 0.12 cal/g°C from room temperature to 55°C, jumps +2 cal/g at 55°C, and continues the linear decrease with a slope of 0.096 cal/g°C from 55 to 90°C. The data suggests that isotactic stereochemical impurities in the atactic material are able to crystallize at 55°C which is theTgofi‐PMMA, and therefore must be associated asi‐PMMA islands in a rigid atactic‐syndiotactic PMMA matrix. This also suggests that the &bgr; transition in PMMA may be associated with the isotactic impurities