  We report on the first results of experiments to measure the recombination
rate of hydrogen on surfaces of astrophysical interest. Our measurements give
lower values for the recombination efficiency (sticking probability S x
probability of recombination upon H-H encounter $\gamma$) than model-based
estimates. We propose that our results can be reconciled with average estimates
of the recombination rate (1/2 n(H) n(g) v(H)A S $\gamma$) from astronomical
observations, if the actual surface of an average grain is rougher, and its
area bigger, than the one considered in models.
