  We present a spectroscopic study of the shape of the dust-extinction law
between 1.0 and 2.2um towards a set of nine ultracompact HII regions with Av >
15 mag. We find some evidence that the reddening curve may tend to flatten at
higher extinctions, but just over half of the sample has extinction consistent
with or close to the average for the interstellar medium. There is no evidence
of extinction curves significantly steeper than the standard law, even where
water ice is present. Comparing the results to the predictions of a simple
extinction model, we suggest that a standard extinction law implies a robust
upper limit to the grain-size distribution at around 0.1 - 0.3um. Flatter
curves are most likely due to changes in this upper limit, although the effects
of flattening due to unresolved clumpy extinction cannot be ruled out.
