The influence of changing the block length of the polydimethylsiloxane block on the structure/property behaviour of two poly(ether ether ketone)-polydimethylsiloxane (PEEK-PSX) multiblock copolymers and their amorphous (non-crystallizable) ketamine precursors (PEEKt-PSX) was investigated. In the precursor block copolymer form, as well as the reduced amorphous and its corresponding semicrystalline form, a microphase morphology was noted from transmission electron microscopy (TEM) studies. For the solution cast precursor or the compression moulded reduced amorphous systems, the materials displayed two T(g)s at ca. -130 and +145 degrees C, indicating strong phase separation. It was found that the room temperature tensile modulus of the PEEK-PSX multiblock copolymers with amorphous PEEK blocks of (M) over bar(n) = 4000 increased by 800% as the PSX block length was decreased from (M) over bar n = 5000 to (M) over bar(n) = 3000, i.e. the PSX mass fraction was decreased from 56 to 45%. This increase in stiffness resulted from developing a more continuous phase of the PEEK block component. After crystallization of the PEEK blocks, the room temperature modulus of both of the PEEK-PSX copolymers increased by 200-500% in contrast to their amorphous forms. Relative to the PEEK homopolymer of approximately the same molecular weight, the melt crystallization half-times for the multiblock copolymers were increased by a factor of 500 for the PEEK(4K)PSX(3K) system and by a factor of 1000 for the PEEK(4K)PSX(SK) system. A dramatic dependence of crystallization behaviour on thermal history prior to crystallization was observed in both microphase-separated block copolymer systems. Block copolymers thermally crystallized from the glassy state obtained a distinctly higher degree of crystallinity at a much faster rate of crystallization than the same copolymers crystallized from the melt at identical crystallization temperatures. Copolymers crystallized from the glass attained maximum levels of PEEK crystallinity of similar to 40%, while those crystallized from the melt attained a maximum level of crystallinity of only 5-20%. The rate of crystallization for samples crystallized from the glass was similar to 30 times greater than that for samples crystallized from the melt at identical crystallization temperatures. Possible explanations for these observations are presented.