The h eats of combustion of two sam ples of polybutadi ene and four cop olvm ers of butacli ene and styrene pr epared at 50° C (122° F ), and one sample of polybutadiene an d four copolym ers po lym eri ze d at 5° C (4 1 ° F ) were m eas ured in a bomb cal ori m eter. Th e stn ene contents in th e co polymers vari ed from approximate ly 8 to 55 weigh t percent.. A va lu e for t he heat of polymerization (or co polym eri zation) was calculated for each sample by comb ining th e exper im ent al valu e for t he h eat of combustion wi th other known t hermoch e.mical data. Th e m ean valu e obtai ned for th e h eat of polymerization (-tlI-J p 0) of polybutad lene IS 17.4 kcal per mono m er unit, and the values obtain ed for t he heat of copolym eri zation var ied fr om 17.1 to 17.7 kcal p er m onom er uni t . No app er ciab le differe nce was found betweep ti? e heats of copolym eri zation of t he 5° C and 50° C pol.vmers. Th e h eat of cop oly meri za t IOn can be repr ese n~ed a s a fun ction of styr ene conte nt of t he p olym er b.v a n equatIOn of a form den ved theor etICall y by Alfrey ancl Lewis, a lthough th e co ncli t ions und er whICh th e polym ers were form ed were su ch t hat som e of the as~u m pt i o n s und erl Yinp: t h e de ri vatio n of th e equation wer e not satisfi ed..