Diffusivities of bovine serum albumin and dextran were measured in matrices of concentrated succinylated collagen (SC), a rigid-rod biopolymer, and hyaluronic acid (HA), a more flexible biopolymer. The diffusional behavior was found to be influenced by the rigidity of the matrix polymer chain as well as by the rigidity and morphology of the diffusing solute. Matrices composed of mixtures of SC and HA showed more diffusional hindrance than matrices comprised of only SC or HA. The diffusional hindrance varied depending on the composition of the matrix (i.e., ratio of rigid-rod to flexible polymer). Electron spin resonance studies using spin-labeled high molecular weight flexible probes indicate that the rigid-rod SC molecules are able to mix with the flexible chains. The apparent interpenetration of the flexible chains (HA) within the rigid-rod (SC) matrix submesh reduces the mesh size in the composite matrix as well as increases the rigidity of the flexible chain.