Block copolymers consisting of rubbery lignin derivative components (i.e. hydroxypropyl lignin, L) and cellulose propionate (CP) segments ranging in size from degree of polymerization (DP) 5 to 60 were synthesized using chemistry described previously. The block copolymers (LCP) varied in relation to architecture and size of component. The copolymers were characterized by solution, thermal and optical techniques, and their effect on the properties of L/CP blends was determined by mechanical tests. The results suggest that copolymer properties are dictated primarily by CP segment size. Whereas copolymers with short CP segments behaved like spherical macromolecules, copolymers with large CP segments had rod-like behaviour. All block copolymers displayed T(m) values, even if CP arm length was as low as 5. Crystallinity was highest at CP chains of DP 60. Copolymer morphologies varied; they revealed patterns resembling dispersed fibrils, spheres and alternating lamella types depending on composition and segment geometries. The block copolymers had surprisingly little effect on L/CP blend properties. This was attributed to the overall compatibility of lignin derivatives with CP.