Clearances of sodium and B<sub>12</sub> were measured with increasing ultrafiltration in two newer coils with new types of membrane support. In all other coil studies previously reported, clearance increases with ultrafiltration did not equal predictions from contributions of convection implying simultaneous decreases in diffusive solute transport with increasing transmembrane pressure. Such deterioration was attributed to masking of membrane surface area by the membrane support, widening of blood channels, and/or channeling of dialysate flow. In these newer coils, clearance increases were very near to predicted enhancement by convection while diffusion was stable in one type and decreased only modestly in the other. Coil volumes assessed with the kerosene technique increased with higher transmembrane pressures similar to findings in previous coils studied. Thus, although coil blood path dimensions are altered with increasing pressure as in older coils, diffusive clearances remain stable and increases in total clearances equal that predicted from convective solute transport. These results suggest that membrane masking is probably the major mechanism for decreases in diffusion with ultrafiltration in other coils and that the problem has been minimized with improved membrane supports.