This study clarifies the contribution to overall osmotic kinetics of colloid osmotic pressure (H) and the interaction of synthetic colloids with the membrane. Solutions (6%) of dextran with weight average molecular weight (MWww) 68 800 (DEX 70), dextran with MW (40 000 (DEX 40), hydroxyethyl starch with MW)(w) (70 000 (HES 70), gelatin with MW)(w) 60 000 and albumin were tested. An osmotic flow cell fitted with membranes of molecular weight cutoff size 30 000 or 50 000 was used to measure time-dependent changes in P for each of these solutions. A linear viscoelastic model was fitted to the curve describing changes to 171 as a function of time. Values of total effective P for DEX 40 and DEX 70 were larger than those for HES 70, gelatin, and albumin. As an index of solute-solvent exchange rate at the membrane surface, these values were in the order DEX 40 > DEX 70, HES 70 > gelatin, albumin. The findings suggest that DEX 40 may be preferable for the temporary restoration of plasma volume because of a heightened initial osmotic force. In contrast, the osmotic force exerted by gelatin is slower to increase but is likely to be longer lasting in vivo as a result of the inhibition of gelatin from penetrating the capillary membrane due to its interaction with negatively charged groups in the endothelial glycocalyx.