Previously, polyurethane (PU)-lignin polyblend sealants from the point of view of adhesion to various substrates, morphology based on scanning electron microscopy, and miscibility based on differential scanning calorimetry (DSC) and C-13 nuclear magnetic resonance (NMR) were studied. In this study the same polyblends together with PU filled with a few mineral fillers were used. Swelling tests were applied to establish the number average molecular weight between crosslinks of the PU and to assess the degree of interaction between various fillers including different types of lignin and the PU matrix. Lignins were found to restrict less the degree of swelling of PU than that by the Sillitin (siliceous clay)-Titanox TiO2 3.17:1 wt/wt mixture. Data based on the swelling method show that lignins do not interact with the elastomer matrix to as great an extent as Sillitin/Titanox filler mixture, a finding which is in good agreement with the former DSC and NMR results.