High-solids anaerobic digestion of sewage sludge is a promising technology, but blocked mass transfer is a key problem. However, the blockage process is difficult to quantify due to lack of direct evidence and data. In this study, a modified diffusion membrane cell was used to analyze the diffusion coefficients in high-solids sludge using iodide ions as a soluble tracer. The total solids (TS) concentration was verified to have a significant influence on the mass transfer in the sludge. The diffusion coefficients decreased sharply due to the decrease in free water when. sludge TS concentration increased from 6% to 12%; the coefficients then decreased gradually, mainly by capillary diffusion, to a lower value when sludge TS increased further to 15%. Compared with the diffusion coefficient in water, the value in the diluted de watered sludge at TS 14.92% decreased almost eightfold, while the value in the digested sludge at TS 15.21% decreased over 90 times. Furthermore, the diffusion coefficients in the digested sludge were an order of magnitude lower than in the diluted dewatered sludge at the same water content, because the digested sludge had smaller sized particles and less free water. (C) 2016 Elsevier Ltd. All rights reserved.