The glass-transition temperature of methyl methacrylate and acrylonitrile block and random copolymers is shown to be sensitive to the structural arrangement in the copolymer. Block copolymers have glass temperatures which are intermediate between those of polyacrylonitrile and polymethylmethacrylate whereas the random copolymers have glass temperatures which fall below that of either homopolymer. Results show that the acrylonitrile block, to a large extent, governs the properties of the block copolymer.