Single crystals of structure II (sII) and structure I (sI) clathrate hydrates were grown in aqueous tetrahydrofuran (THF) and ethylene oxide (EO) solutions. Normal growth habits from the melt are predominantly {111} crystallographic planes for sII, and {110} for sI. Addition of polymeric so-called kinetic inhibitors in very small amounts changed the growth habit of sII to thin, 2-dimensional hexagonal {111} plates. Growing crystals in THF-rich or THF-poor solutions produced similar-looking flat plates. Adding the same inhibitors to the sI system caused rapid small-scale branching of the crystals. The highly branched sI specimens were found still to be single crystals. Higher concentrations of inhibitor were found to stop the growth of the crystals completely, for both sI and sII. These concentrations were as low as 0.1 wt.% at low supercooling. Experiments showed the polymers to be practically irreversibly adsorbed. A physical hypothesis was developed to explain the observed growth inhibition. This hypothesis appeals to a structural fit of the inhibitor polymers on the hydrate surface and strong bonding in specific configurations. (C) 1998 Elsevier Science B.V. All rights reserved.