AbstractThe cocrystallizing behavior shown by copolymers of butene with a number of linear and branched α‐olefins has been investigated by x‐rays over a range of copolymer compositions; degrees of crystallinity, crystal phase changes from those of the relevant homo‐polymers including lattice expansion and contraction and helix modification, and melting points have been observed. Lack of randomness in the copolymers is discussed. Pentene forms a highly cocrystallizing system based on type I polybutene (PB‐I); propylene shows limited, and ethylene no isomerism of monomer units. These three comonomers markedly accelerate the well‐known spontaneous transition from PB‐II to PB‐I, and in sufficient amount cause direct crystallization from the melt to PB‐I. Hexene and octene units can enter the PB‐II lattice, but α‐olefins with C > 8 do not. Butene‐3‐methylbutene is a highly cocrystallizing system; 4‐methylpentene and 4,4‐dimethyl‐pentene copolymers show limited cocrystallization and no cocrystallization, respectively. Linear α‐olefins with C > 5 and the branched comonomers retard the PB‐II PB‐I transition and in sufficient amount stabilize the PB‐II form completely. For comonomers which enter the PB‐II lattice, stabilization is attributed to steric inhibition of the change from an 113 (PB‐II) to a more tightly coiled 31 (PB‐I) helix, but for those that do not, it is believed that the large units act as “stops” at the fold boundaries, which prevent the helical chains being drawn through the crystallites and thus inhibit the transition.